Ash if it is not too far away from you go back & remove the section of pipe you will find the small white cap blocking the pipe we have had a few of these. You do not need to replace the PRV but may be you can alter the pipework so it is how Part G & the MI's say it should be installed & not likely to be much difference in the cost (you shouldn't even need to drain the cylinder as the check valve will hold the water from coming back.
Thanks, I'll go armed with a new valve anyway. I'll check for blockages and if that's all it is I'll adjust the price accordingly. I would have done this when I first went but I haven't met the landlord and had no guarantee I'd be paid. I've sent a price and asked for 50% to be paid first.