personally not heard of a 'support carer'... we have a selection of people who are called our 'support network' - i.e. people who can help us out when we need it - illness, clash of appointments etc.
Those looking after FC need to be CRB checked by SSD - even if they have a CRB from another source (crazy but true)
I would imagine that if you NEEDED a break - the LA would arrange respite at their expense - and that would be other approved Foster Carers - not someone you had nominated iyswim
28 days holiday - boy, they sold that one to you didn't they!!! Or maybe not..
we don't get any 'holidays' i.e. time away from the kids - we do get upto 8 weeks allowance paid inbetween placements, but can't just take a day off cos we want too.
Also, unless legalities/practicalities prevent it, it is looked at very disapprovingly if you choose not to take a FC away on holiday with you.. - in these instances respite care (again, some existing Foster Carer) looks after the kids (at the LA's expense - but then again, you wouldn't get your allowance if you've not got the kids). Unless for the reasons stated above, I personally could not imagine going away without the kids.
Gosh, I would love to have a set number of days "holiday" given to me - whatever will they think of next... allowing us to go off sick?? 
Sorry if that's not helped... I'm not sure of the concept of 'support carer' in those terms you talk about