We reduced our persil tablet usage down from 2 to 1 per wash, this has helped a bit - we noticed that the washing was coming out a bit wet and soapy, and also that the pump does a whirr-whirr sound over and over.
We have had the problem on and off for months really, the reason I posted was that when I cleared this lump of chalk from the top of the filter there was a big improvement:
No whirring pump, and washing not soapy or wet. Spin speed back to normal.
Sometimes the extrernal drain pipe gets full of chalky deposits, which is because here in Hants the water is very hard. I take it off and flush it out with a tap in the garage when this happens. I grew up in Brum and I think the much softer water came from Wales, we never had any trouble like this there.
Before I bit the bullet and stripped it down, we tried a boil and soda wash to clear it - and we do them every few months, it might help a bit, but the problem remained.
Slight correction: I noted today that ours is a Premier 520, not 500 - they are probably very similar.
Why manufacturers make it so hard to access and clear the pipes I don't understand. It isn't helping anyone if we just clutter up landfill with machines discarded just down to a blocked pipe.
I keep telling wifey that she should use a mangle and washboard, but she won't have it.