My speeds with Snap have always been around 1600kB/s download and around 120kB/s upspeed

Last week a new cabinet got installed at my exchange (70 metres down the road).

Since then my speeds have dropped to around 900kB/s and 50kb/s.  Could this be because new houses/customers have been added to the line?

If so is there anything I can do about it?  would changing ISP's make any difference at all?