I will try just one last time. You say "i am probably better off being honest and saying YES", having just quoted "Declaration subject to the rehabilitation of Offenders Act 1974". So check the exact wording of the Act, and you'll see for yourself what you have to declare. Answering the question, however it is phrased, "NO" is being honest, with spent convictions.
In the example you gave about the time before it became spent, saying "NO" would have been lying. Now it is not lying, because the conviction is spent. However it sounds as though you have decided to declare it anyway, which is of course entirely your choice.
It seems odd not to listen to the information you have been given in direct answer to your original questions. So, for the final time:
Could anyone help me here....I have DR10 conviction which is now spent... Do I have to declare this
No.
and will this stop me from getting the job.
Possibly. Declaring it probably makes it more likely.