only down stairs lights wont come on , 6amp fuse switch wont stay up even when main switch turned off on rcd

100% short circuit fault, probably caused by using a too high a wattage bulb in one of the light fittings somewhere? resulting in the wiring being melted togetehr.

6Amp MCB not staying up even with power off? Have a qualified/registered electrician :-) replace the MCB firstly, if its not staying up with power off its more likely to be mechanical failure of the device itself. Sometimes a tap to the body of the MCB with handle of screwdriver resolves it, but I didn't tell you that. However be prepared for a possible further fault on the circuit in question that may/may not be the source of the problem.

You have a short circuit somewhere.call an electrician

In one of your downstairs electrics there is a short circuit, i.e. e.g there is a neutral wire touching an earth wire You need an electrician to trouble shoot and fault fix

The obvious first thought here is that there is a short on the lighting circuit. The lighting circuit is probably not on the RCD protected side of the consumer unit which is probably why switching off the RCD makes no difference. Either way, you probably need to call an electrician (one that can accurately diagnose electrical faults)