I have read a couple of posts about the pangasius catfish recently which claim that the fish can reach size's upto 4ft in a short space of time?
I have had mine for around five years now so am guessing its fully grown at 10-11". What i would like to know is if there are any different types of pangasius and possibly if there is a dwarf variation.

What size tank is it in? If forced to stay in a small area they can easily suffer whats known as growth stunting.

The Pangasius genus contains some huge catfish. P. gigas is the largest at 9ft, and the smaller ones (the ones most commonly sold in aquaria) get to around 4ft. P. hypophthalmus is an example. As for dwarfs, depends on you idea of a dwarf, but im fairly sure no-one would say that a 3ft catfish is a dwarf, this would be P. djambal if your interested.
At 10-11", your fish is stunted from an incorrect diet and/or a tank too small for it.

It has never had any issue with space currently housed in a 200 gallon, when i first got him he was 2" long so kept him in my 50 gallon growing tank until i could introduce him safely into my big tank.
As for feeding if anything he has been overfed as never seems to get full, i feed all my fish a mixture of floating and sinking pellets, prawns, cockles and lance fish but i have also seen it have a go at the mealworms my arowanas eat.

At that sort of size in a 200g tank, it should be a lot bigger and causing you some problems.
The foods your giving it are fine, so it could be another factor. Water quality is a possiblilty.

Or possibly (though less likely) it is one of the species that does not grow as large. Without a picture for ID it is pretty hard to be so absolutely sure about the OP's fish. As an example below is a list of the smaller growing members of the Family Pangasiidae from Fishbase:

Pangasius elongatus - 28.2 cm
Pangasius humeralis - 38.9 cm
Pangasius kinabatanganensis - 23.8 cm
Pangasius lithostoma - 25 cm
Pangasius macronema - 30 cm
Pangasius mahakamensis - 18.2 cm
Pangasius myanmar - 23 cm
Pangasius tubbi - 40 cm
Pseudolais pleurotaenia - 35 cm

Now I am by no means saying it is very likely that the OP has one of the above, but it is a possibility. Also, I have never heard of a Pang getting over 18" in captivity, but I am not that aware of whether that is due to them being a large fish that stays small in captivity, or because of them dying early.

No, your absolutly right Andy, it could be.
I diddent mention them because ive never heard of one in the UK before, and id imagine the price would be sky-high for one.
It could just be a fluke import.

Could be a Pangasius myanmar but i'm no expert! will take a picture to let you get a possible ID. I find it unlikely but possible that i could have got a fluke import.