bb69 - seeding batch won't make a huge difference, not between e.g. C and D. There would only be a short time between these batches crossing the line. I would not compromise the Comrades training in search of a quicker marathon. It tends to be advised not to run a quick marathon after early February, as this eats into Comrades training.
Debra - good advice from McH. I tend to use my London place as an opportunity for a nice jolly run round. I used Shakespeare Raceways as my Comrades qualifier last year - terribly boring running round a racetrack but it was flat and relatively quick.
John - cracking time at Frankfurt, well done and see you in Pen C.