Fly for two hours with an instructor, then take the LPC. The LPC is a minimum of 0.6 logged hours. If you want to remind yourself what is involved in the LPC, it's on page three of the
form.
You can fly all this time with an examiner if you want, or use your favourite instructor for the first two hours. It's up to you.
The only thing that happens as your rating goes out of validity is that you now need two hours
IN ADDITION to the LPC, as opposed to two hours
INCLUDING the LPC if you do it whilst you are still current. Of course once your rating is no longer valid you have to pay for an instructor to sit with you for the time required to get you up to two hours in the last twelve months, and you are paying for (at least) an additional 0.6 hours flying. Since you can anticipate the LPC by up to 90 days, there's not much reason why you should let it expire.
Once five years have passed since your rating expired, there are additional training requirements set by the CAA, and also CAA charges on top.
Just so as you know the terminology the CAA calls an LPC taken within the rating validity period as a revalidation, and after the rating validity period a renewal.