From what I can gather, (although I will ring them tomorrow and check it out now you have mentioned it), I can have it done next visit for free, but Im unsure whether I can hold off until neutering time, I dont really see why not, it was an offer with the vax course, it didnt stipulate a time scale - I will ring tomorrow and find out for definite.
The normal chip is free, if I want the thermo chip I need to pay an extre £9 to offset the cost.