Obvious answers aside. En-suite has had a pungent sweet smell for weeks now. Smell gets all round the upstairs when it’s bad. It has a shower, but gets little to no use-but I make sure there’s always water in the trap. Toilet and sink both drain freely. I’ve plumbers Maited the pan connector to within an inch of its life. Bleach/drain cleaner doesn’t seem to help. It seems to be worse when the main bathroom/shower has recently been in use.

Did have a problem with the internal vent pipe in the loft where the pvc pipe meets the flexi hose before connecting to the roof vent-it was too long and created it’s own u-bend so wasn’t breathing. When I took off the clips a fair amount of water came out-but just assumed the water had come in from outside-however I’m starting to doubt that theory as no water got into the nearby extractor vent. Would it be possible for water to come up the way and sit in the flexi pipe if there was a blockage further down? Starting to do my head in!

Something dead. Always has that slightly sweet tone.

Try covering the drains in the sink/shower/bath and see if the smell disperses. The traps there may be dry/not working

Hair trapped in the shower drain?

I have the same issue, only when the weather has been wet then warms up suddenly. Never figured it out.

Bath not getting used much and traps dried up? I always try and keep the plug in the bath and/or run water down the bath every couple of days

Failed air admittance valve on top of the stack maybe. Aka a durgo valve.
If it vents intp the loft then it might be that.
Have you tried a tiny bit of bleach in your traps as well, just avoid viakal if you do this as bad things happen

It has a shower, but gets little to no use-but I make sure there’s always water in the trap.

Is that why perhaps, a u-bend full of stagnant water? Chuck a kettle-load down followed by a toilet bleach chaser.

Durgo valve-tell me more. No obvious valve on the soil vent pipe at all. It just vents straight into a roof mounted plastic tile vent. I’ll take a look at the main bathroom vent pipe-see if it’s a different design.
Can you put a durgo in the loft space?

If there are almost horizontal waste pipes and the smell originates from them, a foaming drain cleaner may help. As the title implies, it froths up to fill the whole pipe when you pour it down the plughole and attacks any gunge that has formed above the waterline.
Ignore the instruction to “leave for an hour.” Let it stay overnight before you rinse it away.
Good Luck

https://www.drainagepipe.co.uk/soil-and-waste/what-is-a-durgo-valve/

There’s all the info you need.
I wonder if you have a tiny leak under the bath or shower thats not showing through but is damp all the time

Durgo valve?  I always thought they were gurgle valves 🙂  Must get my ears cleaned out

Stop pissing in the shower ?.

Worth bookmarking this website:

https://www.dri-pak.co.uk/tips/

Specifically, Bathroom Drains & Plug Holes:

https://www.dri-pak.co.uk/tips/bathroom-cleaning-tips/