Thank you all... great help
45a is the main breaker.. ring main 16a down and 16a upstairs,
Summer House has Hot Tub 10a (6a Heat and 4a pumps etc) 2 lights, one radio, fridge 1.5kw oil heater with a 2kw fan heater for boost.(as when required)
Was thinking about putting 2.5mm T&E in a conduit...