You should tell your employer that the police policy is that they will not support applications for disclosure for employment purposes. Say that you've been advised that, instead, the company should apply to the Criminal Records Bureau:
http://www.crb.gov.uk/Default.aspx?page=381
The CRB will run a check on you at 'standard' level. This will produce an official print out which will list all of your criminal convictions EXCEPT those which are spent. (i.e. it won't actually show any convictions).
The CRB check is the route which potential employers are meant to follow. By trying to get you to obtain a printout from the Police National Computer they're trying to (a) save money and (b) circumvent the provisions of an important law relating to personal privacy.
For further information about the Rehabilitation of Offenders Act, check out the various links down the left of this page from Liberty:
http://www.yourrights.org.uk/your-rights/chapt ers/the-right-to-privacy/spent-convictions-and -rehabilitation-of-offenders/index.shtml
Liberty operate an advice line for questions relating to apparent breaches of your human rights (which is what your employer's demand seems to be). Limited resources means that the advice line can only be staffed part-time, so call on 0845 123 2307 during the following hours:
Monday & Thursday: 6.30 pm - 8.30 pm
Wednesday: 12.30 pm - 2.30 pm
Chris