Hi Serena, a good question and not so easy to answer, but here goes.
Assuming you are making, say, four foot beds and 18 inch or two foot paths, that means about two thirds of the area needs a good 2-3" manure. I recommend 6" for smaller beds where there is a need to fill them and smother weeds and grass. You can spread 6" but that is a lot of tons as I reckon five tons would be enough for 3 inches on your beds. But I do not know a source near Frome, ask around for stables, somebody will give you a lead.
You could also buy a 3 ton load of green waste from Dimmer for your paths (at 1-2" thick, will save a lot of weeding, hold moisture and give a nice clean surface to walk on once it settles down) and also to bulk up the manure.
A caveat on my tonnage figures is that thickness of manure and compost is difficult to measure because of lumpiness and it varies according to how settled it is: three inches now is two inches in a fortnight as it loses the initial fluffiness, but that also depends how decomposed it is!
I hope this give you ideas at least, and perhaps it all sounds a lot but it should set you up for a good few seasons with top-ups of about a ton yearly, and you may manage that with your own compost.