Piece of very easy stuff to do. Just google companies house (make sure it's the offical GOV) one, and you just register with them. You need to do a search to check no-one has the name you want - you need a Company Director (usually your OH) and a company secretary (could be you), and away you go. 

If you get horribly stuck, you can contact Business Link who are (usually) very helpful with all sorts of stuff. They'll also talk through if it's worth doing or not. We only did it so people buying stuff from us would think we were bigger and more important than we actually are (and therefore more likely to buy from us), it wouldn't really have given us any protection from anything as our business loan was a personal one, and we were too small to take credit etc from companies, so there wasn't a huge amount of point being a Ltd Co really, other than the customer confidence bit. My father is an Accountant, so he did all our returns etc for the Ltd company for free - if you don't have access/want to pay for one, I wouldn't recommend it off bat as if you forget to file/leave a box unchecked etc HMRC love to fine you.

If it's just OH doing contract stuff one his/her own, do they really need to be Ltd though? Surely just self employed is enough - just call your local tax office to register, and they help you do everything, a lot cheaper and less hassle really.

You don't have to do anything other than keep track of income and outgoings and pay your tax at the year end.  I personally hate admin so I used an accountant, but many simply do the self assess stuff.  Accountant charges vary - shop around.

From the start, keep every receipt (those for the biz AND those that 'could be' - write on the back what it is if not specific) and a copy of any pay - invoices etc. and organise them by month.  Even if you get a bit of stationary from ASDA - tally it up and write the biz stuff on the receipt - it all adds up.  At the end of the year all outgoings will offset income to get the earnings. 

As long as you don't get too cute with claiming, the tax man will leave you alone - things like car mileage, phone(s) etc. 25% personal use.

Check Nat Ins - you don't have to pay a stamp for all years of your life.

Happy to help