how can i fix a leak on my worcester 28cdi rng?

There is no nut to tighten, it's a push fit o'ring. Replacing it involves draining the boiler (CH and HW side), removing the filling link assembly then removing the plate heat exchanger exposing the o'rings.
Advise replacing the HW o'ring too, generally speaking, that's the one that usually goes hard and starts leaking.

Whilst carrying this out, have plenty of paper towel (or similar) to protect the pcb.

Carefree
February 2011