If you exceed the £150 then the excess must be reported via P11D. Even worse than that, it is not the total costs incurred minus £150 that would be reported but the cost of the functions that exceeded the limit. So if you have two events in the year at £80 and £90 each then you would have to report the lower value function in its entirety. This is an easy mistake to make when looking at staff entertaining, you have been warned.
One of the peculiarities of this regime is that everyone attending an event is entitled to £150 per annum. So employees' spouses, for example, have their own annual entitlement too!