I have been offered a BT Boost Engineer by PlusNet but I do not know what that entails. By Boost, I assume that the transmitted power will be increased!
Hi there Nolly. A Boost visit is the same as an LLU SFI, basically the engineer is tasked to resolve your broadband issue, and the original brief was to 'make the end users experience of broadband as good as possible'

.
The big difference is that a Boost visit is up to and including the CP's router, and the engineer should be carrying a spare CP router to replace yours with *if* required.
Hope this helps.
I'd say go for it. I might even have a read through this thread