I agreed to buy a property with the same solicitors,albeit at different offices, acting for the vendor and myslf.After getting a structural survey and search the vendor informed my solicitor that they now wanted to let their son buy the property.My solicitor then contacted me to say they were prepared to buy my survey and search . I delivered these to my solicitor. Six weeks later I am informed they will not pay for the survey but I have just found out from my surveyor that they purchased their own survey based on my one at a greatly reduced price. How do I stand legally with either my solicitor or ex vendor?

I agreed to buy a property with the same solicitors,albeit at different offices, acting for the vendor and myslf.
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My solicitor then contacted me to say they were prepared to buy my survey and search . I delivered these to my solicitor. six weeks later i am informed they will not pay for the survey but i have just found out from my surveyor that they purchased their own survey based on my one at a greatly reduced price. how do i stand legally with either my solicitor or ex vendor?

I don't know how it's viewed there, but in the U.S. it would normally be considered a possible conflict of interest for the same law firm to represent both buyer and seller.

It appears that the seller, by offering (through your solicitor) to buy your survey, and then using it, entered into a contract to buy the survey from you. At the very least, by the doctrine of quantum valubant, you would be entitled to the amount they saved by using your survey.