I know this will probably go down like a lead balloon because it has a limited audience, but the Easter holidays next year for Welsh and English schools are not the same in certain areas (please check with your local authority first!). This means that the price for the first week of the Welsh holidays (29th March - 2nd April 2010) is less than half that of the following week.
I've just booked a mid-week break for 4 adults and 2 children in a woodland lodge at Elvedon for £319. (Longleat I believe is £399) The following week is £669 (Longleat £769) for exactly the same break.
These places fill up fast, especially the closest to South Wales, Longleat Forest. So get in early.
Also, you need only pay a £100 deposit now, the remaining balance to be paid in Jan 2010.