Although that's a reasonable size tank I think it's probably too small TBH, at the size they are they require a minimum 460 litres of water, which is approx 100 imperial gallons.
Is your tank 88 imperial gallons or US gallons? And is that 88 gallons of actual water that you have in there, or is that the total tank volume if it were filled to the top?

I really appreciate the advice, thank you. We have 350 litres of water in the tank as it stands....

That's a bit small for two turts of that size, especially two males, and I suspect it's at least a factor in the aggression. I would keep the divider in place and fill the tank as much as you can, can you create above-tank basking areas that would allow the tank to be filled right to the top?

i believe so.... it's hormonal.....territory the lesser one is trapped... he can't go to the other side ot the lake and be safe from the other.... just my feelings on the matter....

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I'll certainly fill it up as much as I can and get creative with a higher basking area. It may prove a little tricky as I have four cats and won't take any chances. It seems strange as they moved into this tank in October and now have much more room than before? Could the change in environment have affected them? Do you think that ultimately I need to separate them?

you could try removing the dominate one for a week or so and let the submissive one establish the tank as his and then re-introduce the mean guy...see what happens.... i don't know but it might be interesting to watch how their behaviors go???

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Well separation would certainly stop them fighting! But I'd try a few other things first, especially if separation means rehoming one of them as that may not be so easy.
As it's a corner tank it should be fairly easy to build a basking shelf in the back corner, surrounded by walls on two sides they won't be able to escape, then you can fill the tank as deep as you like. You may just need to add a perspex wall along the front edges to keep them in.
Then with a divider down the middle of the tank and through the middle of the basking area they won't be able to fight. If your filter is adequate for the water volume then you won't need to change that, and the same basking lamps can be used to illuminate the whole basking area, no need for two of everything.

the sooner you split them up or give them more space the less likely they are to hurt one another. like graham said an above baskin area is a great idea, am currently in the process of makin one, should be finished by tonite so if you want ill pm you some pics.

where do you get all them =S

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That would be fantastic - I'd really appreciate that - thank you. My email address is mrssamsimpson@hotmail.co.uk