Im insured on my partners car and ive had a rta, which was not my fault but have no witnesses. will he lose his no claims bonus

So your partner is the policy holder and you're a named driver on his policy, right? If you cannot prove that the accident was not your fault, then he will lose his NCB. Unless the third party accepts liability, it will be recorded as 50/50 which to be honest is equivalent to a fault claim.

When that happens, this can take a lot of time btw (mine took 2 years to settle!), you will have a record of 1 fault accident on your records but if you have paid for repairs, you may get 50% of the expenses back.

So the best thing you can do, is make your case strong by showing how the accident was not your fault but the third pary actually. Diagrams, photos or anything that can help to prove your innocence can be forwarded to the insurer.

Btw, what exactly happened in the accident?