you could go to your panning office and find out what footings are required and dig them yourself to save you some money but £25k is horrendous money for the amount of time and materials required, sounds to me you have a day and half removing and toothing out exhisting brickwork, 1 day digging and pouring the foundions, 2 days for a bricky, 2 days for a chippy (if that) then depends on what roof covering you opt for but shouldn't imagine it'll be anywhere near that, you not know any people you can call into help you?