Shaun - definitely run for a charity that has some personal link to you or someone you know well, even if you can find another charity that is asking for less. People will give generously if you have a convincing reason and a personal motive for raising money.
I ran my first FLM in memory of my sister for Cardiac Risk in the Young (CRY), as did another sister. This charity raises awareness and funds screening and research into heart conditions that kill young fit people (the kind of thing the Sevilla footballer died of in August).
Together, my sister and I raised nearly £11,000 that year. It all snowballed as people were inspired to help out, even with our son's nursery organising a mini marathon which the BBC came to film with Colin Jackson that was aired during FLM 2005. There's no way we could have raised that much if it weren't for a cause that meant so much to us. I ran for them again this year and raised £2,700. Luckily next year I have a ballot place as I can't keep relying on the same people's generosity.
Find a cause that you can believe in and you'll make the target somehow.