a11 and Paul:
Just found this on the IND's website so decided to post for the benefit of future applicants.
When to apply
You should make your application before your existing permission to stay in the UK runs out. In categories where you have to complete a certain period to qualify for further or indefinite leave to remain in the UK, you shouldn't apply more than 28 days before you complete the relevant period.
The date of application is that on which you post it or have it accepted in person at a PEO. Your application may be delayed or refused if you apply too early in a category in which you have to complete a set period - for example, in categories leading to settlement in the UK. This advice is particularly relevant to applications in the categories for which you have to use Forms SET(M) or SET(O). Please see the guidance notes in those forms for more information about this.
Your immigration status while your application is being decided
If you make a valid application before your permission to stay ends, your existing immigration status will continue until your application is decided, even if the decision is not made until after the end of your permitted stay. If your existing visa or other permission to stay here allows you to work, you can continue to do so until your case is decided.
Applying by post
Please make sure that you send your application to the address given in the application form. Sending your application to a different address will delay it.
Recorded and Special Delivery post provides you with proof of posting and helps us to record the receipt of your application. We strongly recommend that you use it and retain the Recorded or Special Delivery number for as long as your application is with us.
source:
http://www.ind.homeoffice.gov.uk/applyi ... plication/
