According to "The Ultimate Guide To Roses" (Phillips and Rix), page 37, this rose is a seedling from R. Gallica Officinalis.

I planted this rose out of a lack of good places under a large rosa moyesii bush on the north side of my house and forgot about it. About three years later in June I unexpectedly came across a huge most beautiful flower coming out of the dark. First I thought it was a Dis moi qui je suis, because I could not remember where I planted what rose. But the later has much smaller flowers and slightly less pink and with a deeper purple red. The Lord Scarman has since grown high into the Moyesii bush and up the wall. The flowers are spectacular in size and colour. The back of the petals have a silvery-purple appearance. The scent is not very strong, actually I can only detect the scent very occasionally. It should be the rose for difficult dark corners and walls. The branches are rather weak and it likes something to grow into, may be a trellis as well. It is very vigorous and the foliage healthy with large dark green leaves. The flowers do very well in rain. They do not become brown and mouldy. Later the single petals fall down nicely and leave a clean hip.
Dis moi qui je suis needs much more sunshine and doesn't like strong neighbours. Otherwise it does form a very nice bush with wonderful flowers. Unlike other old roses of that deep crimson purple, the flowers of these two roses do hardly fade or become bluish. Still Lord Scarman is my favourite.