James, if the Humbrol product you are refering to is "Mattcote" it will be fine over Future. I have used it in that order with no problems. Just make sure you leave the Future to cure for a couple of days and stir the Mattcote really well before use. I usually thin to 2 parts Mattcote to 1 part Humbrol thinner. For me the 3 keys to success here have been;
1.Plenty of drying time for the Future.
2. Stir the Humbrol product really well, (To disperse all the white matting agent).
3. Just spray enough Mattcote until you have the Matt effect, then stop. If you keep spraying more and more on, you will just get a white chalky haze.
Cheers...Snowy.