See
http://www.caa.co.uk/docs/175/Lasors_Section_H.pdf .
Basically you'll need both to attend a seminar and to pass a FI renewal Skill Test. Not cheap - but if the seminar provider is also a FTO approved to conduct FI renewals, you may be able to obtain a discount.
Note that if your rating has expired by more than 5 years, you'll need to apply to the CAA to have it renewed once you've passsed the seminar and test; less than 5 years and the FIE can do it.