If you've only got 2 hours use neat fairy liquid with one of those sponge/scourer thingys.If it's raining,(it is here)do it in the rain rather than use tap water.Water out of the sky is much better than out of the tap.(no added alluminium etc)
Cheers
at the risk of stating the bleedin' obvious.............
it is possible to buy stuff called Hypalon cleaner and it is fabulous. Brings it up like new. Pretty whiffy and not cheap. Don't know what the active ingredient is. It's worth the expense. Comes in a 500ml tin with s screw top.
Might be a bit late for this post but "Magic Sponge" from the £1 shop will do the job extremely well and not leave that gritty residue that things like CIF will leave and take a lot of effort to get rid of