From what you say, it seems that the Court set 28/11 as the date for him to pay the �10K. If so, he is not yet in default of the judgement & he is trying to get a Variation Order to pay �20 per month. If he gets it before 28/11 and then regularly makes the �20 payments each month on time he will not have defaulted and you could not get a charging order.
So yes, you should write to the Court (recorded delivery & keep a copy) objecting to his proposal to pay �20 per month. The Court will then decide whether to vary the order and if so how.
If it is not varied by 28/11 then you should go for a charging order immediately (Court office should be able to tell you the procedure) unless you want to make him bankrupt.
So far as bankruptcy is concerned I suggest you go here:
http://www.insolvency.gov.uk/bankruptcy/whatis bankruptcy.htm
This is the Insolvency Service's own site. There is an enquiry service phone number & e-mail address at the bottom of the page. This is a very helpful service and should be able to tell you how to go about making him bankrupt.
Please note that the site Ethel put in a link to says you could only bankrupt him if the debt is unsecured. If that is correct you could not bankrupt him if you had first got a charging order. I am not sure whether that is correct, and suggest you ask the enquiry service about it.