Interesting, thanks for the insights! I'm definitely looking for wider than the 23s (thanks hamster for the offer) - currently using 32mm Panaracer T-Servs which seem tough, but feel really slow on the roads. And they are right at the limit of my brake clearance so I should probably go a bit smaller when I replace them. It looks like the GP5000s also come in 30mm so they might be ideal. Hmmm...
I work as a wheelbuilder (at DCR Wheels). We sell so many of these tyres to all sorts of people. Everyone loves them and we have to constantly restock.
Thanks everyone! I've now got hold of the tyres I was originally after (28mm 4 Seasons) but given the responses I'll definitely be keeping an eye out for a set of the GP5000s in 30mm.