Timber,
Just had a word with my better half, who used to work in HR.
As we have limited knowledge on the situation (I.E we dont know how large your company is and if you have a proper HR department) its difficult to know what your companies own policies on this are.
She has reccommended that you have a look at the ACAS website:http://www.acas.org.uk/index.aspx?articleid=2174
On there you will find a free phone number. Give them a call mate.
She has also mentioned that if you really arent happy with the situation and that you need time to find out your "rights" so to speak, then you are able to postpone the meeting until such time that you are clear in your mind that everything is above board.[:T]
Its also well worth seeing if you have a copy of your companies policy and proceedures. READ them! Make sure you go in knowing how your company is supposed to run things, as if they slip up, you can pick them up on it. However it also helps
you to NOT slip up and end up being picked up on something[

]
Good luck mate, hope that helps a bit[:W]