You can buy genuine diaphragm repair kits 43028-1015, currently £10.24. This is the part that often splits and lets petrol into the carbs and floods the engine which then hydro locks and basically destroys the engine
When you rebuild the fuel tap, use the genuine kit as above, and also replace the spring 92081-1171 £1.75. Some people then suggest stretching the spring a little, to increase the pressure on the diaphragm. The original spring rusts and collapses. another cause of fuel leak.
I think that some people don't replace the spring, which then gives the impression that the diaphragm is still leaking, making them replace the whole tap.
I also flatted off the aluminium parts of the tap, using 1000 wet n dry paper on a sheet of glass, with water, to ensure a perfectly flat face on each piece. I covered the faces with a black ink pen/sharpy. and then rubbed them down lightly until all the black ink was removed.
The tap now works perfectly, but I still check for leaking fuel before starting up he motor!