I thought it was worth replying on this topic, albeit it's been an age. I finally fixed the problem, as you suggested.
Filters were all cleaned / replaced.
Took off the cyclone section (be careful if you do this, not to break the bin latch).
Jet washed the cyclone section. Inspecting it with a strong lamp, I could still see obstructions deep within the cyclone cylinder (hair, felt, fluff, dust). My jest wash approach might have worked had I tried it again with a bit more precision, but instead I got a skewer and manually cleared the blockages out. This was fiddly, but in the end they all cleared up. Strikes me as more than a slight design flaw to have something that is so small and easily blocked, but I suppose it has been in service for quite some time.
I then reassembled.
And hey presto the machine now works as if it's new. Suction is completely restored and the noise has gone. It's now picking things up with ease.
So my recommendation if you can hear a sucking sound at by pass valve (and you've checked pipes for blockages) is to work you way through the cyclone unit and unblock the top and THEN the exits at the base, with a jet wash or skewer.
Thanks to you here on the forum for your help. I got there in the end and am delighted to have a machine that works. Expect I'll be ordering a few new parts from you soon - was holding back in case I could not fix it. Now it's done it's time to get a few other bits and pieces to make it as new.