'1. I will miss them too much - espcially DD3 who is only 2'
Ok that is a bit all about you - but 2 is very young to go far away for a long time.
'2. He is iffy on maitanance right now due to 'cash flow' issues but can afford the flights? (villa is free)'
Not unreasonable at all - he need to pay for the shoes and baked beans before the lovely fun stuff.
'School turns a blind eye to 10days 'off' per year - this uses up the entire allowance in one go meaning I have to take them away in holiday time - which I can;t afford'
I don't think it's a great idea to take kids out of school for that long anyway so not too much sympathy on that point BUT if you are doing it then September is the worst possible time.
'4.He's taking his GF (who he left me for) (yes that IS unreasonable I know but still...)'
Yes it is. Very.
'5. We agreed in the summer hols we would EACH take 2 weeks leave to reduce childcare costs (£150 a week for holiday club) - this would now not happen'
Yes a wise arrangement. If he isn't doing his bit then he should pay the £300 for the holiday club. The childcare allows both of you to work - NOT just you!
Overall no I don't think you are being unreasonable to allow only a week. Keep chasing the maintainence!