Washing Machine Drainage Issue

Hi,
I have a Diplomat APM 8703/2 washer Dryer. After any wash the water appears to be draining from the washing machine fine, but during the draining cycle it fills my kitchen sink up with about 2 inches of water.

After a approximately a minute the water drains away fine from the sink too.
However, the next day I will find about 2 inches of water sat in the bottom of the washing machine drum. I dont understand where its coming from as after the washing/drain cycle has finished there is no water in the machine. I believe drainage problems are usually caused by a clogged filter pump…

Things I’ve tried so far are:

Putting caustic soda down sink and boiling water
Raising the corrugated drainage hose where it connects under sink(this has maybe reduced the amount of water left in the drum, but I dont think I can raise it any higher, maybe get a longer hose?)
Unfortunately, because its an integrated unit the filter pump cover is blocked by the kickerboard/plinth. I’ve tried removing this using a suction cup and managed to unclip one end, but then it gets wedged under part of another unit and the other end is blocked by the other walls skirting board.

Any advice would be very much appreciated.

Thanks,

Andy