Yesterday we were returning from Barmouth to Dolgellau. Just about to cross Barmouth Bridge and a series of touring motorbikes are coming off the bridge.
Should I shop to the Police, if so how?

If they shouldn't be there then yes. Driving is a privilege and comes with certain responsibilities.
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or
https://www.north-wales.police.uk/contact/minor-incident-reporting or https://gosafesnap.wales/
If they caused you nor anyone else no harm or damage then what’s the point? Remember that without trespass there’d be no countryside access. Leave em be if they were sensible.
So you report every horse rider or mountain biker that rides on footpaths then?? live and let live
Yup that's exactly how society works best - opt out unless someone gets hurt. If it doesn't effect you it's not your problem.
Use of motor vehicles outside of permitted places is an offense under the Road Traffic Act for a reason it's not a simple case of trespass. If you can't grasp the reasons why then there's little hope for you.
Problem is that once it gets discovered then the big gangs start turning up making life hellish for all.
Difficult quandary.
I’m in 2 minds, they were being sensible, but there were loads of young kids on bikes/walking the bridge. It’s not the widest and potential to fall in to the sea.

I was up a coed y brenin last Sunday and had a group of I think enduro motorcycles follow me down the steep rocky bit down to the bridge., they were courteous enough not to try and get round me till the bottom and then shot off up the hill. I have seen these guys on motorbikes at nant y arian a few times bur never at coed y brenin on the mtb trails
Should I shop to the Police, if so how?
By posting their numberplates on a public internet forum maybe.
Yup that’s exactly how society works best – opt out unless someone gets hurt. If it doesn’t effect you it’s not your problem.
Nope. If it doesn’t affect you nor anyone else it’s not a problem by its very definition. But you go and report it. But make sure that every time you see a mtb on a footpath you report that too. Oh and make sure you loop in anyone driving over the speed limit. And anyone beeping their horn after 23:30. That’ll learn em.
If they caused you nor anyone else no harm or damage then what’s the point? Remember that without trespass there’d be no countryside access. Leave em be if they were sensible.
It might be quite a good analogy if they didn't have somewhere more suitable to ride.
They’re not doing anything wrong, it’s always been open to motorbikes unless something has changed recently.

It might be quite a good analogy if they didn’t have somewhere more suitable to ride.
Define suitable.
about 264,000 miles of paved road
Nope. If it doesn’t affect you nor anyone else it’s not a problem by its very definition. But you go and report it. But make sure that every time you see a mtb on a footpath you report that too. Oh and make sure you loop in anyone driving over the speed limit. And anyone beeping their horn after 23:30. That’ll learn em.
In your case I'll change my post. No hope is probably more accurate.
They are not going to do that crossing again as they were obviously on tour. Comlpetely different situation to crossers riding on bridleways that are likely to do it again.
They should have gone up to that toll bridge to cross, which is further up and a reasonable trek, so that is probably why they crossed at the old railway bridge.
As above, motorbikes have always been allowed over the footpath.
The bridge a bit further up the estuary is open to cars as well as pedestrians.
Maybe the toll signs have been removed now, it's around 4 years since I was last there. Check youtube, there are videos of motorbikes crossing the bridge.
Nope. If it doesn’t affect you nor anyone else it’s not a problem by its very definition. But you go and report it. But make sure that every time you see a mtb on a footpath you report that too. Oh and make sure you loop in anyone driving over the speed limit.
I know this isn’t really the thread for this, and many (most?) of you will disagree with me on this.
But yes, speeding is a problem. Driving standards are poor in the country. People in cars and trucks often treat people on bikes or people on foot extremely poorly.
Choosing to drive above the legal limit, and doing so deliberately, is not ok. If it’s not a choice and you haven’t done it on purpose then IMO you shouldn’t be driving at all, because you failed to be in control of the lethal machine you’re in, or be aware of your surrounds - including the current legal maximum speed.
If you’ve chosen to break the maximum permitted speed to save a few seconds journey time then you’re part of the problem of road deaths and near misses.
Not that speed of the primary cause of all crashes: I know that isn’t the case.
But choosing to ignore the rules that make traffic as safe as is reasonable erodes the importance of & attitude to all the other rules - safe parking choices, safe stopping distances, safe use of phones, respect for other road users. If everyone tried hard to be a good driver, obeyed all the rules as best they could the roads would be safer, better environments with likely (IMO) less stress, congestion and harm.
So yes, obey the limits. Choosing to go faster is selfishness, and nothing else. Sacrifice a few seconds of your time to make the potentially lethal thing your doing safer.
Driving is a licenced, insured, regulated privilege that needlessly kills many many people as a result the selfishness, and as a result of the attitude that “speeding’s ok”.
It isn’t. Follow that rule. And the others too.
Be a decent human.