you need to have a rotary double pole isolator in the loft, not a main switch type.
If it's a TT supply then you need to cut into the tails before your main switch isolators/earth leakage devices. Using 16mm tails you would then feed a garage sub board with a type B RCD (as available from Sibberts) and a 16A breaker. If you don't need to have an RCD for reasons under 1771 then don't use one. I believe that although the 4000TL (which I think you are using) is tranformerless I believe it has an earth leakage device that satisfies the RCD requirement for not having electrical seperation (but not if one is required for other reasons under 1771). I don't use them so someone else may be able to clarify this. If you do need an RCD it should be a 100ma type B.
Don't be tempted to use the household RCDs for reasons explained at length on other posts. If you are TNCS or TNS then still use tails before the main switch and fit a non RCD garage board if no RCD is required.
Powervan is right, the whole birds nest really needs sorting out!
