Well first thing to do is to get the invoice. And you look like you're going to fall within the CIS scheme by hiring a subbie, even if the job was for a private householder. Just as well get the registration over with - leave it six months and you'll have racked up a few thousand in fines if you are caught.
Depending upon whether your subbie is himself registered under CIS - similar to having one of the old scheme's 715 cards - you should have stopped him tax at 20% or 30%. Can you recover that from your subbie? You won't know whether to stop 20% or 30% until you're registered yourself, whereupon you phone a helpline to give your subbie's NI number, and you'll be told stop 20% or 30% tax. That's the tax you'll be sending to the Revenue by the 19th of the month, with your return. The subbie gets it as a tax credit to set against his eventual tax bill for this tax year (that's the year ended 5th April 2009).
If you cannot recover the tax element from the subbie then you'll end up having to pay it yourself. Chances are your subbie won't be on the CIS register - he'll soon tell you if he is - so you'll need to aim at 30% tax. Ask him nicely for the 30% tax back, so that you can pay it to the tax office on his behalf. If he's smaller than you, tip him upside down and shake his wallet out.
If you cannot quite remember the exact date you paid the cash to your subbie, September or October, then October would be in your favour because it would give you until 19th November to register and file a return. You'll be asked when you paid him - note that when he did the work is irrelevant and has no bearing - it's when you pay him that counts. (So he might have done the work in August or September but get paid in October). I've seen canny contractors avoid the possibility of late registration fines - that is where you are registering for CIS after having paid someone - by saying they are waiting to pay their subbie (they reason - quietly and to themselves - that any cash that has been given to the subbie was a personal loan between workmates, to be squared up on the first payday, as soon as the CIS registration is through).
Note: I'm not suggesting you do that - no way, no sir, not me. Just making the observation.
Final point - the class 4 your brother in law is talking of relates to being self employed (you pay class 1 national insurance if you're an employee. If you're self-employed you pay class 2 and class 4 national insurance on your profits. They're just an extra form of tax. That's why we have one of the lowest tax rates in Europe - because we have additional taxes called national insurance. Tony Blair spin!). Now if you've started self-employed, you have 3 months to register (I'm fairly certain it's 3 months from the end of the month you first became self-employed, so if you started 1st July you have until 31st October. Late registration for self-employment: yes, yes... another fine!
I'd talk to your subbie first and recover 30% from him. Receipt wouldn't go amiss, although once you have his tax you actually issue a receipt to him. Then you're ready for registration. Is there no way you could talk to your brother-in-law's accountant? Notwithstanding the warnings that I gave earlier about accountants and bank managers etc being legally obliged to report you for any tax evasion, you might be better off with him rather than going to the Revenue like a lamb to slaughter. Especially on a Monday - oh, those Revenue people are extra-mean on Mondays.
Good luck Captain, and get back to the thread if you're stuck. It's a lot to take in, because it's no good just giving you snippets. I think you'd be better off with an accountant on your team - there's always the guys in these and other threads who seem to cover London, Wales, Wiltshire, Manchester and some other places I haven't heard of. I'd offer to help, but this Goose is about to fly South.
EDIT: simultaneous posting - MAXINE we overlapped some advice - apologies; I think we're in accord
