Any recommendations? I’m looking at Schwalbe Big Ben or Marathon plus but they seem a bit pricey for what I need.

I guess tyres from Maxxis and Schwalbe start at £20 or so.
If that’s too much, I haven’t seen many fat city tyres in 650b, but there are plenty of commuter tyres:

E.g. http://www.decathlon.co.uk/city-3-hybrid-bike-tyre-650x35a-id_8328679.html

Never even thought of checking Decathlon, good shout thanks!