Seen a NC23 with 52km's on the clock for £1000, its got mot and is supposed to be straight.

Should I be put off by the mileage ? I thought It might be too high ?

Have had no luck finding a cb or cbr for the better half

Has the bike got alot of service history, old mot certs, etc... if it looks in good condition and has lots of history, then i don't see a problem, unless it's been thrashed on the track, raced etc.
My mate has an nc23 and it has got roughly that sort of milage on it and he has had it for quite a while and it's been fine.

my NC29 clocked over 80K klms a few weeks ago.
rule of thumb with ANY bike.. look for service history, old TAX disks, MOT certs, receipts for oil/bearings etc, and HPI check the bike for peace of mind.

my 29 was just about to tick over 75000 and is still running sweet, even after my crash

I've gone past 110000 Km now - I bought it with 97k or so on the clock, came with an extensive service history indicating the oil and filter had been changed every 5000 kilometers (give or take 1k).
Any excuse to boast the most devaluing feature of my bike - although now it's also starting to look 110k

condition more important than ages.
the CBR400 mills are tough and will deal with high mileage without too much trouble so anything up to 100,000kms be fine.
as with any bike regular service and care are the key :D

in terms of 400s, i have been lead to belive the CBR would be a safer bet that a VFR with hign milage.
the reason for this is the V4 is a bit of a bastad to work on, where as the CBR engine is not near as compact (IL4) and supposedly easier to work on. i dont mind working on the V4 tho!!