I'll say to you what I say to everyone who asks about tax matters on here.
Speak to an accountant.
That is all.
Easier to go direct to the tax man 
http://www.hmrc.gov.uk/calcs/esi.htm
HR people deal with employment. Accountants are for figures. Taxman does it all 
The taxtw@ is very difficult to pin down to give an answer when it isn't clear cut in print.
It's a long story (which I won't bore you with) but through them being evasive, I only paid my January tax bill last week.
It was a lot of money, it was in their interest to give me what I wanted but because there wasn't the answer to my question in their script, they wouldn't answer it.
An accountant could say 'it's not cut and dry, you can't do this, but you may want to do that but I would suggest the other...'
I do agree with you CC re: going direct, but only if the question is relatively simple. But when it comes to tax, the answer is rarely simple.