See LASORS C6.3 for full details of the NPPL (Microlight) and the differences between 'unrestricted' and 'restricted' licence privileges.
The NPPL 'Licence Allowances' document at
NPPL gives details of the requirements needed to add a SSEA Class Rating to a NPPL (Microlight).
LASORS C6.2 describes how a NPPL with SSEA Class Rating may be upgraded to a JAR-FCL PPL(A) with SEP Class Rating.
Bear in mind that if your Microlight training has been conducted by a Microlight Instructor who does not hold a JAR-FCL FI (Aeroplane) Rating and only the additional SSEA training has been conducted by such an FI, then you will need to top up the total training time with a FI authorised under JAR-FCL.
For full licence privileges, see ANO Schedule 8. But it isn't easy reading as it's all in 'shall not unless' doublespeak!