Hi, thought I should update this - and apologise for not thanking for the advice!
In the end I requested a set of Planet X Super Roubaix bib longs, Large.
They are very comfortable and warm, and a good fit.
They are also falling apart after two rides, despite not having put them through the washing machine yet - seams are coming undone about the place with thread hanging out. Heard plenty thread popping when taking them on and off, despite being gentle.
Hopefully I just have a dodgy pair. I'm sure PX will replace them.
Anyone else had similar issues with these?