I bought a digital vernier for around £6 delivered from ebay, was almost exactly the same as the £40 machine mart one, except it had a larger display, works perfect had it about 2 years now, so don't pay over the odds, I prefer the non digital ones but these are more expensive so didn't bother.
Maplins had bore scopes on offer a few months back for around £35, with a screen built in, seen one demo'd and it was excellent clarity.
tabetha