Seriously, budget for a gimble head. If you are splashing out that much on a lens a few hundred to give it the best connection to the tripod should be done without a second thought. Keeping everything balances makes the whole process so much more enjoyable and the big lens feels like a relative lightweight - I wouldn't be happy with my 300mm F2.8 perched on top of a ball - let alone something even heavier!
A few people use the manfrotto 393 and rate it very highly - this is the cheapest option at £120. Next you can look at the Jobu heads which for that weight are coming in at £320+ depending on needs.