Bob,
I have been through such a search for a grave in Sunderland myself, and the graves are not easy to locate on your own, even if the reference number is known. Especially if there is no gravestone.
There is, however, an excellent "cemetery hotline" service available, and they located the grave and marked the location, and we arranged to meet at the cemetery where they showed my the grave. Excellent service !
The number is (UK number) Cemeteries Office on tel : (0191) 553 1687 where the staff will be pleased to assist you. This is probably your best bet for a result by speaking to them direct.
Best of luck with it all.