Anyone know if this is allowed or do I need to bolt an extra rating on to my EASA PPL?
See CAP804:
3.13 Exercising the privileges on Microlight aeroplanes on the basis of an SEP(Land)Class Rating
The holder of a UK issued licence or any Part-FCL licence with an SEP rating, may, subject to differences training on the appropriate class with a suitably qualified instructor, exercise the privileges of their licence on microlight aircraft. However, any experience gained in microlight aircraft cannot be counted towards the flying experience necessary to maintain the full SEP or TMG privileges.
This also applies to the LAPL(A):
1.7 An LAPL(A) does not have an SEP rating, but does have SEP privileges endorsed, therefore may be rendered valid for Microlight aeroplanes, subject to satisfying the above (differences training) requirement.
Seems very strange they don't put down weightshift as a mandatory differences training to be honest.
They do, but it's poorly worded in CAP804:
3.11.2 Microlight pilots wishing to convert between weightshift and 3-axis microlight control systems, or to a microlight with more than one engine, shall undertake differences training given by a flight instructor entitled to instruct on the microlight aeroplane on which instruction is being given. The training must be completed, recorded in the pilot’s personal flying log book and endorsed and signed by the instructor conducting the differences training.