has the boiler only just been fitted? if i remember correctly on a vokera linea, the condense pipe is a black plastic tube, it should go into plastic waste pipe, not copper or brass. if it has been fitted recently with no condense pipe i suggest you call the person out imediatley and get him to...
like you said, you don't know whats wrong as you have not been there, and i wish i could fix every boiler by just checking the pump and bleeding!
Ravenheats are one of the worst manufactures i've ever known, the amount of times i've phoned the manufacturer and even they have been wrong!
i agree with rob884, either the new valve is faulty or there is some reverse circulation going on, swaping from a y plan to s plan would not make the problem go away, just cost you a lot with two valves, pipework and wireing alterations. If you can see check that your return from the cylinder is...