Ok, after a few phone calls today, this is what I have learnt.
My local dealer / dealers within the group can and will look at the car but want to charge the £150. There is no escaping this it seems. You want the car checked, you pay. You don't pay, you don't get the car checked. Fabulous service.
On calling the dealer I bought the car from, I was informed that they would look at the car and check it with no payment required. The words went something like this.... "We would not attempt to extract money out of a customer who has issues with a vehicle under warranty"
I explained that unfortunately, I am at the other end of the UK to them, and so bringing the car back was not reasonable / possible.
Now, I explained their comments to my local dealer, but they said that this is not policy, and can't comment further on it. So people, it seems that when you buy an Approved Used Mercedes Benz, and, you have their tier one warranty (which is all singing and dancing if you read about it on their website) you will need to shell out circa £150 to have the pleasure of getting MB to look at your car. From there, you need to "trust" that they'll be honest with you and offer a refund if it transpires the car does require work which is "ALL" warranty covered. If not, you'll not see your £150 again, plus, may end up having to pay to repair a car that you thought had comprehensive cover.
To say I am disappointed in this would be a huge understatement. It's nowhere near the sort of cover I though I would receive, and even further from the customer service I stupidly assumed one may receive when buying an approved MB. I have no idea what other manufacturers behaviour is like with regard to warranty issues such as this, because for years I owned BMW and didn't required any warranty work, because the car was faultless.
From here, I plan to speak to the dealer I bought from, and ask whether they would cover the diagnostic fee. If not, I'll leave it. I just cannot bring myself to pay to get a cars problems checked when that car has a valid warranty.
Finally, I received a Private message / conversation from a member, but cannot seem to respond to that. If a Mod sees this and can look into it for me, I'd appreciate it, as I'd like to be able to contact this member.