How can I fix my Hotpoint FDW85 - "AL05"?

I have a Hotpoint FDW85 which is just over 5 years old.

It has now started showing "AL05" (Clean filters according to handbook) a minute into any cycle. The filters are clean.

I can overcome this by switching off the water supply until "H2O" comes up, resetting it and restoring the supply. Start the cycle and it starts washing as normal.

Hotpoint have been no help whatsoever, typically, as it's now just out of warranty.

Can anyone advise which part is or has failed (ideally with part number), so I can run a normal cycle without this infernal code appearing every time? Have no problem repairing this myself, just need to know which part to fit!

Many thanks in advance