If an IMC Rating has expired by more than 5 years, you will need to complete 'training at CFI's discretion' and then fly a full IMC Rating Test with an Examiner such as a UK/FE(PPL). Your form FCL 99 (that's the one used for the test) and logbook will then need to be sent to those awfully nice chaps in the Belgrano who will put the Rating back onto your licence; I think that the fee would be £64.
But if it's less than 5 years expired, you merely need to revalidate it by flying the revalidation test with an Examiner; assuming you passed, the Examiner would sign your Rating page there and then - nothing would need to be sent to the CAA.
Once your IMC Rating heas been renewed or revalidated, it's valid for 25 months. But to fly under IFR in the UK FIR, you do indeed need to have FM-immune VHF navigation equipment.
[ 27 August 2001: Message edited by: BEagle ]