here's a good one for you!!!!!

Why not just connect the expansion vessel to the capped off pipe that used to be the flow from the disconnected boiler? Very simple to do, and it will bring the neutral point to the optimum position and stop any cavitation.

THANK YOU, THANK YOU, THANK YOU, for all your good advice.
I now have a nice quiet, warm house to relax in, well done everybody for the great help you've been.

Here's what I did:

Installed new pump Grundfos UPS32-55, new flexible 1.5" pipework either side of the pump, full sytem bleed as I replaced a couple of TRV's that were suspect, moved pressure vessel to inlet side of pump before boiler.
Nice and quiet now sitting back relaxing with a nice cold Belgium Beer.

Thanks again chaps job well done!!!!!!

Chris hutt.
Sorry about that previous post. I had confused upstream with downstream and didnt look at your diagram.
I imagined upstream to be on the outlet side of the pump.

That's OK sider. I've been known to get muddled too. I hope the diagrams and explanation were helpful.