Basically my speedo doesn't work , the chip inside is burnt, I bought another one which has a different plug socket and wondered if there is an adapter which could plug in between the different sockets to connect them?

nope just got to wire it in your self

a common fault iirc?...

Yea, this is happening a lot. Would be good if someone (maybe even you Ryan582 since you've just started working on it) could document their process of changing the speedo to an aftermarket one for current and future members to benefit. I'm still waiting on edge for mine to go and would love to follow a tried and tested guide when it does eventually decide to kaput.