Ok, latest news. Have double checked height. Lowest point of guard sits about 10mm higher than lowest point of sump so shouldn't be any issues there. Also the prototype guard is continuing to rack up miles under my Golf. Took it out in the wet today and can report zero belt slippage issues which is encouraging. Couple of little issues have arisen though. One is a small rattle/buzzing but I think I've already traced that and it's an easy fix. The other is that, although it stops water getting at the belt, it does collect the odd splash from puddles etc. Therefore a small drainage hole may be required. The trick then will be to stop water going in through the hole as well as out!
S.M.