adding a sink and hot water to cloak room?

[mausmaus]

cloak room currently only has a toilet in. no sink.
space would be big enough to add a small one.

is there such a thing as a small water heater that we could install so we can properly wash hands or even guests could use it to wash?

rest of the house is gch/combiboiler.

You can buy electric inline water heaters for around £50. Not sure what the deal is with wiring them... Whether they need their own circuit or can be wired into the mains. Either way, I'd get a couple of quotes for plumbing your hot tap to the combi... If it's a comparable price to keep the system simple, that's what I'd go for.