I've known Ken from M/C Hobgoblin for nigh on 40 years and, whilst he may stock a good range, its not the greatest and certainly not the cheapest. I'm surprised nobody's mentioned it but probably the best range of mandolins in the world can be found at TAMCO in Brighton. Trevor's stock is second to none and he should be able to fix you up with a decent instrument for anywhere between £250 and £10,000 +.

I speak as nothing other than a satisfied customer. If you don't know the place check out the website - its like pornography for mandolin fanatics!
Ray