I just found a website that allows you to type in a number that starts in 087 (which are really expensive to call) and get the equivalent that is much cheaper or even free to call.
I wish I had found it earlier as I've spent £10 in mobile phone credit this week trying to contact my phone company Pipex, who have put me on incoming calls only even though I paid my bill on saturday, it's all due to their computers being down for nearly 5 days. Hence my angrily typing this from work, I could have called them for free if I'd had the other number sooner.
The website is called SAY NO TO 0870 (Click the title of this post or the link to visit the site)and is as easy to use as a search engine, you just type in the number or company name and it gives you the alternative number. Just wanted to share that and promote the site as it's a really good idea and it's very nice of them to offer it completely free.