Zanussi ZDT 6252 DW not draining properly

Not an answer but a similar problem. Our Zanussi ZDT 6252 DW has worked fine over the years, with the occasional need to to flush out the waste pump when it greased up. Recently however it has stopped draining completely. Just to confirm the waste pipe forms a high loop up to the counter-top before connecting to the U-bend. I disconnected the waste connection at the U-bend (guessing the pipe connection was blocked) and pulled the waste pipe from under the counter top to flush it through. I disconnected the waste pipe from the DW and flushed the pipe (even extending it to get a clear view along its length) and there was no blockage. Confused I reconnected the pipe to the DW (without forming a high loop - so that the entire pipe was at ground level), tried a quick wash and it drained instantly? I did several more wash-throughs and every time litres of clear, warm water drained into my bucket. Satisfied I reconnected the pipe at the U-bend, careful to reinstate the high loop and guess what - the DW wouldn't drain? Three attempts at a quick wash and the DW filled with water and cut-out on the drain cycle. I disconnected the pipe at the U-bend once more, removed the high loop and it drained instantly.

My question - is the pump just getting tired and has lost the power to push the waste water up to the counter-top and out? The pump clearly works if the pipe is at ground level and the DW still washes?

PS. Despite using a cleaning tab and half a pint of vinegar there is still a musty smell in the DW.

Any help would be much appreciated.