when you say compatible with MM what exactly do you mean?
If you want OS mapping with your position shown on it, you've got three choices.
1)Satman 10
2)RA adventurer
3)PDA with GPS antenna and memory map
If you want actual compatibility you've got number 2 and 3 from above.
However, something like GPS babel can convert a MM route into a track that a garmin etrex or quest or whatever can show but you don't get the basemap behind.
You can pick up a RA NAvigator reasonably cheap off of ebay and download the MM application. If you've already got the MM maps this is the best way forward. If you've got to shell out for maps, then look at a mobile phone running viewranger
I've got the RA fitted behind the headlamp, and all the 1:50k and 1:25k for the UK and France plus the aerial photos and road map. On top of that it's a pukka sat nav, so if you need to head back to the van, you can get to the road, turn off MM and navigate using the proper navigation tool
Good thread on it here:
http://www.pocketgpsworld.com/modules.p ... c&start=15