Flicked across to the pseudo ILS whilst on the 28 ILS into Blackpool yesterday, and it was giving me half-scale "fly down" deflection whilst established correctly on the real glideslope. Badness!

The most likely explanation for this is that your SkyDemon glideslope was set to a different value (in degrees) than the actual ILS you were tuned to. That, or your GPS altitude was quite different to your actual altitude, which can of course happen (always cross-reference your GPS altitude to your pressure altitude).