Siemens washer/dryer problem

Hi, I’ve got a mysterious problem with our Siemens WXD1260 washer/dryer. Please bear with me because I’m a bit of a DIY numpty. We had it installed 7 years ago in our new fitted kitchen and it’s been pretty reliable until a few weeks ago, at which point it stopped drying, leaving the clothes wetter than at the start of a dry cycle, or stopping during the cycle with a F04 fault (supposedly signifying a pump or outlet hose blockage). I called out a domestic appliance engineer who confidently told me there was no problem with the machine, and the problem lay in the way it was installed; the machine’s outlet hose is connected to a 4ft extension hose so it can reach the trap under the sink, and he told me that the extension hose was too long for it to drain properly.
Since then though, I’ve been investigating further. Maybe the guy was right, because I found that just draining through the machine’s hose into a bucket, it still dries ok; in fact having thoroughly cleaned out the extension hose, it also works draining through both hoses into a bucket. However, when I connect the extension hose back into the trap under the sink, it stops working again. There’s no blockage there, the sink drains perfectly. And, when a dry cycle fails, even with a F04 fault, if I immediately put it onto an “empty” cycle, it drains ok.
So, has anybody any bright ideas? Maybe the installation isn’t ideal however I never realised at the time and it worked fine for long enough. I also know that maybe I should be thankful for getting 7 years reliable service from a washer/dryer, given the bad press they get; however given the evidence in front of me I’m not convinced buying a new one will be the answer. Do I need a domestic appliance engineer, a plumber, a new machine, or all of them?
Any advice much appreciated!