Indesit PWDE 8148 S, F15 Error code.

My washer dryer flashes F15 when draining. The code apparently means ”F15 – Water is not being pumped out and a flood condition could occur. Check that the drainage hose is raised to the recommended minimum height and that the pipe is not blocked”
So yesterday I removed the drain hose from the drain pump and checked it for blockages, I removed the hose from the sump to the drain pump and removed a few coins and half a pencil, I checked the filter which was clean then I put it all back together again and the fault was still there.
This evening I put the drain hose into a plastic bin rather than the outlet pipe to the outside drain then put the machine on a rinse cycle to observe what it is doing.
It drains with good pressure from the pump for about 10 – 15 seconds then shuts off and flashes F15, if I switch it off then back on and press start it will drain again but switch off with the fault code again after the same amount of time. I had to do this three times to drain the machine.

Tomorrow I am thinking I might pull the drain system apart again and give it all a good clean and look for any more debris in the system and also check the wiring and the pump.
Is there anything else I should be looking at as a possible cause? The drain pump doesnt seem to be making any strange noises and seems to have good pressure.

Sods law our insurance lapsed a while back and we didnt renew it, once this is fixed I will be reinstating our insurance.

Any help on this would be greatly appreciated, Im a plumber retrained as a motor mechanic so I have tools and should hopefully be able to sort this once I know what to look for.

I also pulled this off the net for F15 but I dont think it applies in this instance,

”Dryer element relay
Check pressure switch, wiring and module connections

Also check the dryer element and thermostats for good measure”

Rob.