, I am thinking of building
a cupboard around the boiler. Are there any regs to do with this? Any
advice on how much space to leave around, ventilation, etc?

I also have a Suprima which .....snip.........
..And I've also enclosed mine in a kitchin unit. Made sure to drill
plenty of holes above and below. Seems OK like this. Apart from that
cutting out of course! Good luck

Dunno about clearance, but ventilation wise............Building Regs
document J, Part 3 "Additional Provisions for gas burning appliances with
a rated input up to 70kW (net)", diagram 3.2 specifies one opening at top
& one at the bottom, each 1000mm2 per kW input (net) - that would be ~
12cm x 12cm for 50,000Btu rated balanced flue boiler.