"I thought it was just to say......." - A reminder to anyone to make sure you know what you are signing and if you do not, then ask someone who does.
As mentioned above a Collateral warranty creates a contractual link directly between you and a third party - usually you would only be liable to the party that contracted you to do the work, however by signing the collateral warranty you can now be sued directly by the other parties specified on the warranty!
@Philipmcg - Don't invoice or accept the £1 for consideration! I've come across this - it creates a contractual relationship between you and the developer, where there was previously none (I am assuming you have not invoiced or contracted directly with the developer!). Its sometimes used as a way of sidestepping a collateral warranty where another party (in this case the developer) wants to be able to sue you later on directly if something has gone wrong - so DON'T Do it! Unless of course they are holding up any payments to you, in which case you need to take some professional advice on pursuing what you are owed without the need to pay the £1.
Also - entering into collateral warranties may not be covered by your insurance policies, unless specifically agreed - as this creates an additional liability over and above that which would normally apply!