Foxmoth - do you mean renew or revalidate? There's a big difference now. If your FI rating is still current then you need to revalidate by doing either the Seminar or the check ride as long as you meet the experience requirements - 100 hrs instruction in the last 3 yrs including 30 in the last 12 (of which 10 teaching IF if you have no applied IF restriction on your instructor rating).
If your rating is out-of-date or you don't meet the experience bit then you need to do a seminar plus a test flight. The standard rate for an FIE for the test flight is £170 plus the aircraft and any personal expenses (ie travel from miles away). The test flight will probably last around 1 hr 30 mins and then there's the ground work (including a 30 min lecture these days).
Hope that helps.