@LeeDerbyshire 

Hi,

According to my own experiences using extension cords to connect a Powerline adapter to a wall socket generally reduces the performance of Powerline adapters. 
A shorter extension cord should have less of a negative impact than a long extension cord.

However, whether using a shorter extension cord would be sufficient to solve your Internet problems you can only test by yourself.

By the way, make sure whatever extension strip or cord you use to connect the AV600 to the wall outlet does not have any kind of "noise filter" or "surge protector" built into it.
 

  @woozle Well, it turns out that the worst-performing extension did have surge protection.  As expected, plugging the AV600 directly into the wall gives the best performance, and plugging it into a 1 metre long single-way extension reduces the performance by a small amount, which is acceptable given the practical problems of working with the unhelpful layout of our rooms and furniture.

Interestingly, some wall sockets work better than others, although the working bandwidth only varies within a range of about ten percent.  The largest difference is between upstairs and downstairs sockets, probably because they are on different mains circuits (in the UK we call them 'ring mains').  I will need to do some more testing.