Hi,

I had to do just that last Monday due to volcano-related misconnection.
I will assume you have cleared the 'Connections' security and you are in the transfer desks area (also where the Star Alliance lounge is); go back to (Connections) security and explain to the agent that you need to go to arrivals, they will open a small door at the end of the row of metal detectors. Walk against the flow of incoming passengers, through Customs (in reverse) and past the stand where to dispose of liquids and gels, around the corner, take the elevator up one floor and you will end up back in Arrivals, where they separate EU / non-EU passport control. Clear passport control for the second time and you will be through to the baggage claim area.

NB 1: as you do this, expect a few funny looks from officers and security; slow down, smile and tell sincerely why you are doing this (these things happen) rather than just briskly walk past without saying a word and raise suspicions. This specially applies when you leave the baggage claim area without your luggage, or barely a carry-on - explain why to Border Agency officers.

NB 2: as you initially transfer through connections, they will read your passport, take your photo and give you a little sticker barcode, which normally has to be scanned again and your photo deleted at the time of actual boarding (UK and Ireland connections, I assume). Remember to show this barcode to security when you re-enter the check-in area the second time (and again explain your situation) so they cancel that one before they scan the barcode on your actual boarding pass instead.

NB 3: let me know if the NB 2 above is not very clear, I realise it's hard to explain

Cheers,
J-