If you download the Examiners record for the test you will see exactly what you have to do.
http://www.caa.co.uk/docs/33/FORSRG1157.pdf
Download the SRG1102 to get a new 5 year licence issued; for this you will also need a valid medical certificate.
If its more than 5 years since your SEP rating expired it will have to be re-issued by Gatwick but, if you got it less than 7 years ago you could just be within the 5 years and save a fee.