Having done it in the dry (August time 3 year's ago) and also in the dry (September) but, after our fantastic summer last year there was plenty of boggy, muddy sections full of puddles to make one glad for the MTB (front suspension only) and knobblies.
Start at the Roehampton Uni and trip along through Richmond Park, along the river past Hampton Court is a very pleasant ride, though you have to be aware of runners, dog walkers, fisherman, and some very nice pubs that appear far too early on the route!!
The route down to Shere has to be ridden very carefully, because having a spill on some of the narrow descent will involve some pain. After Shere (and after a pint!) there is a muddy section through a wood you'll eventually come to (well, it was muddy last September), and on some of the ride you'll be doing some section on roads. They're not that busy.
The Downs Link section* that starts at Cranleigh runs down to Bramber, and the official route will take you through Bramber round to Upper Beeding, thence up to the Downs from a steep chalky path. Alternatively, walk the bloody thing or, instead of cycling through Bramber, take the cycle track route** that runs along the river (East side – your left facing South) and pootle down to Shoreham, then cycle along the coast road of Shoreham High Street heading towards Brighton, after a couple of miles you'll see the Shoreham Port Authority (bluish-sh coloured building). Turn right there, and wiggle your way round to cross over the lock, which is much more interesting than continuing along the main coast road. You can then cycle along the much quieter coast road by the sea (alongside industrial units/boat loading quays) which will bring you out by Hove Lagoon. At this point you can choose to join the main road into Brighton or cycle on the Pedestrian/cycle-shared pavement.
The route will take longer than you think, which I think can be used on most rides. At least they can with me. Last year we took around 12 stupid hours, only because of fart-arsing around with members of our group "Let's stick together chaps!" getting punctures, and veering off-piste on occasions – even though we were using our previous maps – it's not so easy when you don't have certain key points indicated by signage or spotting other riders in the distance "Follow them chaps!"
I can always email a PDF of the map to you if you like, but it'll be around 6.5 meg's worth.
*Bloody boring to be honest, as it's mostly straight with hedges 'n' stuff obscuring one's view to left and right and again, to be honest, an alternative offroad route from here down t'Bramber would be preferable.
** We can do this 'cos we live here, and did an unofficial ride on our own using the maps we saved from doing the ride 3 years ago with BHF.
