In answer to your original question, there is a very easy way to find this out, just ask.
I would always advise people to call creditors directly and hint that there may be a cash influx coming. Simply indicate that you want rid of the debt and ask them to put their best offer in writing. The mistake that is sometimes made is to show your hand too early, offer 33% and they will want 40%. Offer 10% and you will find that 33% is suddenly very acceptable. My advice is to NEVER offer anything, just have a general chat and dangle a carrot. If you like the offer then do it, if you don't then don't.
Reading between the lines, I suspect that you are on a DMP, so if you clear 1 or 2 creditors with a one off payment then the rest will get an increased monthly payment, which is good, but you may find that the rest will then become a lot more eager to do business at a better rate to you.
There is no hard and fast rule to this, but to give you an idea, I recently cleared £15k with £6k, on another case creditors would not take less than £11k. It is a very inexact science but, generally, the longer the DMP has been running then the more amenable the creditors are.
Hope this helps.