Hi John, I'm with Julian on the self build if time permits, as you've a large concrete slab laid, was it reinforced and damp proofed, could you lay a one block high base onto it, damp proof, then stud frame on that and Bob's not by chance a Builder Friend .. or Tommy Walsh, now that was a build with his Shop..
I had to buy in the Shop and I have HD 12x10 the price 3 yrs ago was £1200 + something on delivery.. The power for it will still cost at last count before 20% vaaaat £ 500 to 600 depending on power points .. and you need the more the merrier in points.
A look at the Shop I have is selling for close to £1600 now...
What ever you do lets is some pics as you go.
Cheers David