Okay, let's assume that Matelots could be needed on certain SF missions - we can't be looking at a large number of them. RAF guys, too.
The coxwains that drive the Bootneck Rigid Raiders are *Marines* - not Seamen. How about the Landing Craft coxwains - Marines, too?
Marines flew helis in the Falklands, didn't they?
As for RAF, we have the Army Air Corps so I can't see a real need for RAF guys being needed, unless we're to capture and fly off with Osama Bin Laden's hidden fleet of Migs.
Booties work well enough with the army. If we're looking at SF dedicated RN/RAF units, we already have it, don't we?
If "Tri-Service" people are needed for intelligence interpretation, that's a waste of effort for the men concerned. And how many women are going to join because of "Equal Opportunities" legislation?

None of this makes any sense to me.
If I put on my cynical "politics head", I can easily see some sense: cut back on ABF and humour everyone by "creating" a new unit to appease critics. As soldiers we all know the obvious: technology cannot replace the organic seeing, doing, thinking "machine" operating a rifle/MG and holding ground. I don't think any politician who has no military experience should be allowed near the job of Minister of Defence.
David Stirling created the SAS from nothing. The Paras were formed after the successes of the German Fallschirmjagers - for the same purpose. The Commandos were created inspired by Boer Kommandos (initially) to raid the nazis. All were created by serving or previously served soldiers. All had a clearly defined purpose.
Why not expand the SAS/SBS? Introduce a "Ranger" platoon/company in PARA/CDO battalions?
So what's this "Tri-Service" SF unit precisely for? A unit created by politicians with one eye on their purse is hardly a good idea...

Because the politicians have seen that the army can still (just about) cope with the Iraq (mis)adventure, they think that they can afford to cut back. They cut back on those who can't challenge them. They'll never cut back on their (overpaid) wages and expenses. I think our own politicians are as much a threat as the terrorists - if not more.

The politicians would do better to get Paul Daniels to train up a new military unit that could conjure up Weapons Of Mass Destruction or Iraq-based Alqaeda training camps. Blair would like that a lot!
I think someone's been watching the "A-Team" on UK Gold...