I've tracked down a nightmare creak on my gravel bike (sorry kill me) to the bearings on the bottom bracket. There is a very slight rock when you pull cranks on either side. This is majorly disappointing as the bike is not that old and does not come out in harsh weather.

Anyhow, any tips on where to get replacements? I think I need this:-

https://www.westbrookcycles.co.uk/m30_3 ... e_kit-2705

But can't find stock anywhere which confirms my fears that they go regularly.

Assuming they are a standard size ( or measure them with calipers and post up) any bearing shop should be able to order them for you / get them
I just take my knackered ones in and say one of those please. Only American bearings seem to be a pain as often they are imperial

The lack of stock may mean they never break so no one carries them

Two problems:-

1) lack of bearing shop anywhere near me and lack of time to visit one
2) that metal shim for holding the bearing in place, if I bust it on way out I'm stuffed

Acquire a wee pointy metal thing to lever out the circlip - you can buy sets of them in halfords. It should come out easy enough. If it's a creak check the bearings aren't loose in the shell or the cranks aren't loose in the bearing, rather than the bearing being shagged. 'Simply bearings' have a cycle specific section and they do enduro bearings.