Honestly Chris, whilst ChrisR's idea is both pragmatic and excellent, this is the point at which I would cut my losses and get under that floor to expose as much of the 22mm pipe as you can so that you can make on a new coupling. I would use a 22mm Hep2o which is easily demountable.
Then you can run new 22mm copper (or Hep2o) to the pump inlet, with new tees, with the vent coming off lower than the cold feed. Also, replace as much of the cold feed as you can get to with either new copper or new 15mm Hep2o barrier pipe.
A tip, nay, strong recommendation, when fitting a Hep2o coupling onto copper, especially old copper that you might even have to saw...
Firstly, do NOT leave any burr on the outside of the copper. Secondly, don't risk damage to the rubber 'O' ring from any remaining sharp edge, even if there's no burr - the risk arises if you just push on the push-on fitting. This might sound crazy, but instead do this: with reasonably clean hands, disassemble the fitting, remove the 'O' ring, reassemble, push the fitting onto the copper, disassemble, and fit the 'O' ring over the copper carefully with your fingers. Then replace the body of the fitting whilst avoiding pushing the green grab ring further down the tube, and screw up the nut.
If you do this with every Hep2o fitting that's going onto copper, you will never have a leak there.
Whilst meldrew's_mate is being helpful, I doubt that the blockage will be enough to prevent flow if it's been pushed into the 22mm pipework, since it's usually very solid around the tees into the 22mm and breaks up into powder when you poke it.
(The powder is still bad for TRVs, but if you've been running an unprotected system, unknowingly or not, then all these problems have just been waiting to occur.)