What’s a decent small lock that will fit in my jersey pocket? Doesn’t need to be heavy duty as I’m not in city centre crime hot spots?

It doesn’t really exist unless you have particularly large jerseys tbh. Smallest worthwhile lock is probably the Kryptonite New York mini

what he said…

..though, what are you talking about, a lock for all day while at work, or while you nip into the shop for a paper?

Best lock is your house, or a modern car with bike inside of course. If it’s trail riding I wouldn’t bother and just keep your eye on the bike or have a friend do so if you pop into a trail side shop or cafe. Riding to the shops, work, etc., cheap bike and locks that look the part to deter. Locked up next to something that looks more expensive.

Other aspect is if you want to use your insurance if the worst happens, check what they require. Usually particular standards, but they’re all breakable within a minute with the right tools. Just you need them to claim.

Its just for locking the bike up while I pop into the shops

I’d go for an alarmed wire lock. Obviously not suitable for locking up for more than 5 minutes, but at least if someone tries to get away with your bike it’ll be pretty obvious. Lol at the idea of carrying a fahg in a jersey pocket