Do Tanks and trucks get stuck in muddy lakes often? of course they do, "8x8 HET dropping it's trailer and recoverying most tanks" Hmmm no I don't think so, if your refering to the Oshkosh HET then yes it would be up to quite a lot of tasks but not all, however you don't have one attached to every tank Sqn and Armd Inf Coy thats why you have Armoured Recovery Vehicles attached to them, and yes Foden/Man SV® can cope with all wheeled heavy and light plus light Armour and has some limited capability with Heavy Armour.
The RLC tank transporter driver has some recovery knowledge but can only really cope with picking up tank casualites from ECP's and tracks, however they do undertake quite extensive recovery training, but not as much as Recovery Mechanics.
As for Fodens recovering Land Rovers, well why not? we have the assets available it's what they're for then we should use them, not sure if there are any MJ's about these day's maybe sold off as the Bedford fleet is being retired (slowly).
Why have the MoD procured these new Recovery vehicles? well the ageing Foden is just about coming to the end of it's service life as is most of the medium to heavy wheeled fleet, the MoD has procured the Man Support Vehicle and obviously we need the capability to recovery it, you obviously need to go down the route of commonality of spares etc thats why it's based on the Man chassis. These are not "Extra vehicles" these will in time replace the Foden when it gets withdrawn from service.
I'm surprised you see many Fodens being used in the UK on the public roads as there is a recovery contract in place, unless your on the exercise area.
The new Man SV® is cutting it's teeth extremely well in Afghanistan at the moment, it's turning out to be an outstanding vehicle, definately a leap forward in technology.