Is there a really reliable service for hunting down someone in England?

The first sponsored link under your question (when I viewed it) was:

The UK Electoral Roll
Instantly find anyone in The UK -
Unlimited Searches from only £ 3.00
www.electoralrolluk.co.uk 

This is where I would suggest you look next.  People do have the
option to opt-out, but it's something to try.

I followed the advice and searched the UK Electoral Roll for the cost
of GBP 3.00.   I got eleven results - too many people with the same
name.
I do not want to use Intelius - I have had poor experience with them.
Any other suggestions?

I'm rather puzzled by your statement that an electoral roll search
produced too many people (eleven) with the same name - presumably the
name of the person you are searching for. If you don't know which one
of these people (if any) is the person you are searching for, how is
anybody else going to know - short of you giving a description to a
private detective who then follows up each name and address?

1:  I know where this person was employed up to JUNE 2005.
2:  I know what this person's EMAIL address was while she was employed.
3.  I know that her specialty is Marketing and Public Relations.
4.  I know that she is single.
5.  I know her age - approximately.
Is that enough?