for this kind of spend it may be worth looking into companies that could manage your account on a pay per hour basis rather than % of your budget. it could cost £2-3K per month. you could employ someone in house to do it for less than that. heck, i'll do it for 5%.. lol
have a shop around, you'll find some companies do it different ways. my 2 cents anyway.
10% sounds about right. IMO, anyone offering to do it on an hourly basis or <5% is not going to be in a position to make technology, operations, process or personnel investments necessary to ensure that your returns from paid search are better than your competitors.