I have spoken to DynoDaze at Hinkley in Leicestershire and they are up for setting up a day.
@SteveR - To generate interest, I will post up on MR2OC and IMOC for the MR2 crowd and obviously on here too. If the dyno can handle 4WD then I can get a few mates to post up on the other forums too plus I have friends (yes, that's a shock eh) that can post on Aristo, Soarer and other sites.
Once a date is confirmed, Macmillan then send out press releases to the local papers and contact local radio stations asking them to give the event a bit of exposure. There is no guarantee they will but they do try. They will also send along volunteers on the day with buckets, leaflets etc.
After the event, I ask people who have taken pics and video to send them to me so I can publish them on the car sites and Macmillan also put them in press releases and on their site and FB sites.
If you need any other info just give me a shout.