Can anyone enlighten me whether (1) an experienced dinghy sailor/racer has all the necessary skills without formal training to charter a yacht. And (2) if its possible to do so without a Yacht Masters or similar certificate

In general charter companies will want you to have either Day Skipper or Coastal Skipper, depending on the area and your intentions. I don't think it is common to ask for full Yachtmaster. They want your business but they also need to satisfy their insurers that they are only chartering to competent customers.

I'm not clear from your post whether you have previous yacht skippering experience. If not, then "an experienced dinghy sailor/racer" does not have "all the necessary skills". It's not about the sailing, with which dinghy sailors should have no problems. It's about navigation, pilotage and what it involved in maneuvering something weighing several tonnes in confined spaces.   

I'd disagree, it's much harder to stop a yacht. 

Ever tried parking a yacht in a marina in a space that is only 10 foot longer than the boat with a firm breeze blowing you off?

You also need pilotage skills, and although a good dinghy sailor = a good yacht sailor pretty quickly, there is a learning curve (mostly to do with management of the loads).  You probably shouldn't be going through that curve on someone elses £60,000 machine without supervision/guidance!

If you think it is so easy, try it sometime with 30 knots of breeze pushing the yacht around, and maybe some nice strong tide in the mixture. Consider also the fact that the engine pushes the stern sideways at low speeds. It really isn't like "parking" except in easy conditions.

Being a good dinghy sailor will only help you in some of the aspects of skipperring a yacht. OK sailing it around will come quite easily, but that is actually the easy bit of it what you (probably),  won't have much idea of is ;

Maneauvering under engine, with all the complications that have been pointed out,

Navigation,

Electronics, VHF radar 

Emergency procedures

Engine maintenance

As a dinghy sailor you may think that the sailing bit is the important bit - well unfortunately it isn't - the other bits are actually more important.  You can be a piss poor sailor but if you  know the rest you can still be a good and safe cruiser.

I know about this because I have just spent the last 3 years going through the process of aquiring the other skills after 30 years of competitive dinghy sailing.  

I don't know. Where's your club?

1) No.

2) You can charter yachts in the Whitsundays (Queensland, Aus) with no sailing experience whatsoever.

Far more to sailing a larger boat than a dinghy. True that dingy experience will give you the "how to beat, reach, run, etc.", but then what happens when the wind get's up and you need to reef, when you come to anchor, when you reverse and the stern kicks, etc., etc.

Dinghy's are great fun, but if you are skipper of a larger boat then you have responsibilities for the safety of your crew and everybody on board.

This does not mean it is difficult, just there is quite a lot of knowledge you would need that being able to sail a dinghy would not give you.

That said I have done it a few times without any RYA certificates. In fact the first time the guy in charge of the boats forged my signature on my "self declaration of competence" (required by the then Yugoslavian government - I had not realised I was meant to sign the thing as I did not understand a word of the form). However, in practice I had been sailing cruisers and larger boats since age 9 so had experience.

Ian