Hi Clive,
Condensation Cure silicone does harbour moisture and although it should be perfectly possible to use condensation silicone with the Xencast P6 it could be possible that either this particular silicone is holding more moisture than usual. We've tested Xencast P6 with a range of silicones but of course we've not been able to test it with them all.
Is the silicone mould new or has it already made a few parts? - Also, do you wash the mould out (soapy water for example) before use? - If you do, this would be a prime candidate for a problem. It's strange that you had the same problem with the previous FastCast too; that resin has high levels of moisture scavenger in which made it very insusceptible to moisture reactions so I'm thinking there might be an issue with the process somehow.
One last thing, the 'sediment' in the Part A is the moisture scavengers. Are you thoroughly shaking Part A to disperse the sediment before use?
--Matt