Janet, you are in the UK aren't you?
First- congratulations. I know freelancing can be hard work, but on the up side you'll never have to work for anyone again. Well, except clients of course, but they are all so short term it's not the same... You'll never look back.
I recommend joining a freelance photographer's association. There will likely be more than one- Suss out which one supports the sort of photography you plan to do most of. (ie. avoid the wedding one!!) These groups can seem expensive to join but you stand to gain so much in the first few years. You will learn pricing, be able to download contract, copyright and invoicing templates, go to workshops and seminars, meet peers, enter award schemes etc. Then if you have a question there is always someone to ask. Membership also carries a stamp of respectability in the marketplace, and you'll be seen as better than everyone else who knows not much more than how to build a website and find the "P" function on their dslr.
As a stop-gap, just do the Google thing and check day rates on photog's websites. Bear in mind that better photographers rarely advertise their rates but quote on a job by job basis, but I'd expect rates to be in the $500-800
(read Pounds here- in aussie we have different keyboards) range, depending on the work type and what equipment you need. Check the rates they quote, then look at the quality of their folio to see where they fit in the scheme of things- awesome, or embarrassing crap. Mentally compare your work with theirs.
Also note that product work is often charged on a piece rate instead of hourly. Don't forget to charge for digital post production. Consider your company will likely expect a healthy discount on the market rate for offering you the retainer.
As an opening move, why not ask them what they are prepared to pay? They may know the market themselves and you could be pleasantly surprised..
PS- I don't know your market AT ALL, and I'm probably talking out of the **rse end of the planet...