There may be room in The Tomb for four coffins, but I have no way of telling and no access. I am prepared to believe that The Tomb consists of a 'concrete box'. Here is how Charles E. Hardy, author of John Bowes and The Bowes Museum describes The Tomb: Late on the night of Friday, 13th July, 1928, their coffins were brought from the mausoleum beneath the chapel at Gibside and carried by road to Barnard Castle. The convoy arrived there at two o'clock on the morning of the 14th and passed through the dark and silent town on its way to the Roman Catholic Church at the corner of the Bowes Museum Park. The re-interment began about sunrise, without any religious ceremony, in the presence of the Chairman and the Secretary of the Chapel trustees, the architect, and a representative of the Minister of Health by whose order the removal of the coffins had been carried out. ..... After the consecration of the Church in September, the founders of the church were not forgotten: two wreaths of evergreen from the trustees of the Museum were laid on the tomb, where they lay side by side beneath a six inch layer of cement and a heavy roof-like slab of blue grey marble inscribed with their names in letters more remarkable for their boldness than for their elegance.
I can see that Mr Hardy is not too keen on the Bowes's Tomb, and I would have thought that another style of resting place would have been more appropriate. But the simplicity probably arose because the trustees of the Church have always been hard-pressed for funds (and they still are!). But, Terence, whether your granddad's recollections are right or whether Mr Hardy is right, I cannot say. I am hoping soon to visit the new Reading Rooms which the Museum has established in what I think would have been Josephine's apartments if she had lived longer than John, and perhaps I may find an answer to this question.