Thanks for that advice brianj. I put a *lot* more thought into it after this initial question, and the incident of one of them being poorly after being fed some rubbishy treats.
I was relying on the dry food to provide to 'complete' part of their diet, and then the other foods + chicken just as extra protein, and because they enjoy that...but I never came at it from the angle that I could be just diluting the 'completeness' of the diet by adding more meat. It just seemed...more beneficial somehow, and they enjoy it.
I was always wanting to feed them a raw diet eventually, and was putting that off until they were a little older, while trying to keep their diet similar to what they have always been used to while they settle in for the first few weeks...but we've had them a while now, so I did more reading on raw and started that transition.
I found a well-recommended brand of pre-made raw, that is delivered frozen, and started them transitioning onto that, which for one took about 15 seconds but was a couple of days for the other. I was feeding that with a little bit of boiled chicken to entice them in, but that's now stopped (as one of them was leaving the chicken in favour of eating the raw anyway), so for mealtimes they're just eating raw now. And I leave some dry food down during the day for if they get peckish (can't really leave raw food down during the day when they're alone...bacteria etc...).
All seems to be well, and by all accounts they are still two happy, healthy cats :) And I feel a lot better now and more confident in what they're eating. Happy days :)
Link to Natural Instinct if it's of interest to anyone in the UK.
http://www.naturalinstinct.com/categories/cat-food/I bought all three flavours, but haven't moved on to the lamb yet. They're eating beef and venison though so I don't foresee that they'll have a problem with that one either. It seems so much more economical too. I'm not convinced it's easy to find 1kg of any other kind of high quality food for them at £4.50. Even complete dry food costs more for the weight.