Blocked overflow pipe.

No, because the condenser overflow pipe does not come into contact with the heating side and so does not have anything to do with the expansion tank

no if condensate pipework was blocked boiler stops working,so it was not completely blocked,it would not have caused expansion vessel to split.

condensate pipework is for the moisture building up inside the flue due to hot gas getting contact with the cold air and it is very acidic for the copper pipe work, condensate pipework is pvc and has no effects on expansion vessel.

There are 2 pipes that often exit through the wall. 1. The PRV Pressure Releif Outlet that allows excess water pressure to be released at high temperature. Its a safety feature and should not be cemented over, it should be terminated back to the wall in copper to prevent scalding in the event of it discharging. Only if the pipe was 100% Blocked off (i.e. Capped) could it affect discharge as its a appx 3bar plus and a bit of cement wont stop it going back into the house or cavity (If it were capped over and nothing could express there is a good chance, HOWEVER, EXPANSION VESSELS FAIL ALL THE TIME. Cheap materials and a renewable society sadly mean 3 to 5 years is a good life span for one 2. The Condense Pipe, I do not think you are talking about this one, but this could not damage the expansion vessel diaphragm. As a repairs company we are recharging (more often replacing) expansions vessels every week. Conclusion In my professional opinion the two are not related. If The PRV Pipe had been completely blocked and the boiler overheated the pressure could get high enough (Over 3bar) to start damaging the boiler, but it would come with a whole lot of noise like an airplane landing in your boiler. Hope this helps Best Wishes Brandon, CTSGroup