There's one am looking at now but there is a possibility that its done 20k miles without a service should i stay clear or go have a look?

I don't see why it's not worth just having a little look. I suppose it also depends how it's been driven in those 20k miles... And all the other miles...

just knock the money for a service off!! I'd just make sure the timing belt history is spot on

I wouldn't worry about it. That's peanuts. As long as there has actually been oil in it, it will be fine.

Circa £375 - £400 (Max)for a full service and cam belt at a specialist. That includes parts, labour and vat.

If there's no evidence the cambelt has been changed, and by that I mean fully receipted evidence, I'd take it that it hasn't been done, no matter what a seller tries to tell you. After all, it's not his problem if it snaps on your way home.

only other thing that bothers me is that could it be clocked?

Check on the direct gov website for a free full mot history with documented mileage

To be fair, it's possible to 'correct' the mileage of any modern car using a laptop, so even a full mot and service history won't always reveal it.

Always check the claimed mileage is concomitant with the wear to the tyres, seats, pedals and steering wheel etc; not foolproof, but a free check that sometimes suggests a murky past.