Hi Pete,
In all probabillity the rate Woolwich will offer you will be the best option and you are right not to want to change your current tracker as you won't be able to improve on it.
One possible alternative to Woolwich for the extra funds is a second charge loan, but I doubt that would be cheaper than what Woolwich will offer you, but it is worth checking out.
I suggest speaking to an independent
mortgage broker such as myself who will be able to search the entire market and tell you the cheapest way for you to raise the funds. However, I do suspect that Woolwich will be the best option.
Feel free to take a look at my company's website via the _link_ above and if you wish you can contact me for a free and confidential chat on 0845 230 9876.
Hope that's helpful.
Dan