First, sorry for your predicament - first purchase is always difficult. However, I think that you were badly advised to act as you have.
One way of looking at this is that you chose to save money by not having an inspection, and this has come back to haunt you.
The purpose of the inspection that you didn't buy is to see if the machine has any faults that would affect the value that you might pay. Had you taken that option, then the engineer performing the survey knows and accepts that he has a duty of care to you for that very transaction, and that he may be liable for any loss you suffered in that transaction through his lack of reasonable care / skill.
Based on an annual inspection, which is something entirely different, you took a gamble and it seems that it didn't pay-off.
Whether the engineer who performed the annual missed something or not, I don't see his potential exposure to you as being any more in value than the cost of the annual, or more likely part of that cost. Can you show that the condition of the blades at the time of the annual was such that the annual should not have been granted? If you can then the cost of the blade inspection should be refunded as the test was not properly carried out. In addition, as it is a safety related matter, you may be able to raise this with the CAA; given this error should this person have the ability to sign-off a machine as fit for flight?
You did not pay this engineer to take on the risk of advising you for the purpose of underwriting the value of the machine for purchase. You chose to rely on the annual for something that it was not. I don't see any UK court holding the engineer liable for your consequential loss. I would anticipate that the court would see significance in your avoiding the cost of the pre-purchase survey if you asked them to consider the matter.
Seems to me that you are now trying to strong-arm compensation from the engineer based on a risk that you didn't pay him to take on. I don't see that as fair.