Even on very old roses, the graft can usually be easily seen, so if you can, investigate the base of the plants.
Your uninvited guest seems to be very similar to Generous Gardener except in the number or petals - could it be a sport of the variety? If it is from Generous Gardener, will it persist, as in flowers of that form?
What rootstock is used varies across the world, but as your original plants appear to be climbers, in the UK they would probably be grafted onto Dr. Huey, itself actually a rambler, which has deep red flowers (shrub roses are usually grafted to Rosa laxa stocks, which has simple white flowers). Where David Austin might produce plants for N America, would take some digging, or just ask them direct.
