Hi Keith
I've found a local company with the same kind of lorry and am waiting for them to get back to me with a quote.
I've also found a local ready mix supplier that can tip the 3 cubic metres at the roadside, 25m from site, for £275 which is cheaper than I can buy the materials from my builder's merchants for!
They say there is a plasticiser in the concrete allowing for 4 hours workability, but I'm still not sure me and a couple of mates could shift so much in 4 hours! My slab is 150% the size of yours at 16' x 18' x 6"
How far did you have to move your concrete in the barrows? My site is 25m from the roadside. Do you think it could be done in the time?