You'll have no problem mate, I had standard DBS for certain things and got my SIA no problem and had to have an enhanced one for my last job.
If all your offences are in the distant past (and none of them are in a very long list of possible crimes, mostly unheard of ) you'll get in no problem.
I think I have been done for about a dozen (non motoring) offences but got mine straight away.
Good luck for the boring 12 hour night shifts
Cheers, D.