So long as your SEP Rating is still on the front of your Licence, the simplest way is to Renew the SEP at your local Flying School, and then carry out differences training with an Instructor qualified to give Microlight instruction
Alternatively, go to a Microlight School, and carry out the Training /Test required to have a stand alone NPPL(Microlight) Licence issued on the basis of your expired SEP. This may be a bit cheaper, but will only cover Microlights, not the rest of the SEP class.
MJ
