Opened up a paypal dispute and got it settled almost immediately.
To be fair / "balanced" they apologised and said they were just about to pay it anyway, but they had ignored three of my previous chasing emails chasing for payment so how was I to know this?
I know that there was no malice intended and as I said right at the start, I like Planet X and their ideas, but their stock control is awful and caused all the issues. There were problems at each step:
1) they DID have 29" tyres in but it was only one pair, which was shipped out almost straight away.
2) when the 27.5" came in they were put in the 29" bin by mistake
3) when my order was picked the picker just picked them from the bin ticked it off the order but didn't check the bin item itself, which is why I got the wrong item.
4) when they were returned it took five working days for the stock to be re-entered and the resulting refund to be sent for processing. Maybe 5 days acceptable but the few returns I've made to other small online businesses have resulted in same or next day refunds.
again for balance they said they are putting in a new stock control system that will both reduce errors and process returns better, but for the moment it looks Planet X makes bikes first, and is an online retailer almost as an after thought.