Delonghi Magnifica ESAM 4200 steam problem

[JJcoffeeEnjoyer]

Heya folks, I'm having problems with the steamer on my all on one machine, hoping someone can help.

Details:

• It's a single boiler machine
•  It produces coffee fine
• It produces hot water fine
• It's been regularly descaled

Problem description:

• When I press the steam button, then wait for the light to go solid, I turn the steam knob
• The machine produces only tiny spurts of steam, barely works at all.
• Also the pump only moves very slowly - I hear "put... put ... put" instead of the continuous vibration/pumping I get when I make coffee
• HOWEVER if I turn the steam button off, then open the tap, I get about 10 seconds of steam before I get hot water again.

I don't know what this means. I think it might mean either the safety overpressure valve is gone, or there's a problem with the thermosensor on the boiler/generator, but I don't want to replace the wrong bit!
The machine is clearly producing some pressure though, as I do get steam when I turn steam mode OFF, and the coffee is still good enough.

I don't think it's the steam valve on the front, as that just opens the spout and tells the pump to go, right? I don't see how this outlet valve could impact steam generation.

Any advice gratefully received!
Thanks,
Jasper