Delonghi EC820.B water runs thru diffuser w/o the sump but no coffee

Hi there, hope you can help!

I have a Delonghi EC820.B coffee machine.

All of a sudden, my machine has stopped working. Without the sump on, water runs out through the diffuser quite nicely. But whenever I put on the sump with a filter engaged (correctly), water only collects in the filter and doesn't pass through. This is the same whether there's coffee in the filter or not. When I remove the sump, the air pops and a little steam puffs out, suggesting there's good pressure in there. Coffee is wet through.

When switching on to feed water through, it's loud as normal for the first two seconds but then subdues to a quieter, duller hum than usual.

Something about this suggests pressure to me. I have cleaned all the parts I can reasonably get my hands on - the filters were working well, flow was good and then NOTHING, rather than progressively becoming more of a dribble so I figure that's unlikely to be the problem.

Reservoir is full. Steamer passes both water and steam. I have not changed the coffee I have been using all this time with success.

Any help would be warmly received - I'd rather make do and mend in this case!

Thanks in advance.