Two people sitting with their laptops in the same room. The person slightly further from the router is downloading at an up-to-profile speed. The user sitting nearer to the router is downloading at 2-3 mbps less. The two users are not always downloading simultaneously yet the speed difference is the same. Why?
Download from something on the local LAN and see if the speed difference is still present. If so, you are looking at something probably related to WiFi performance rather than broadband performance, which opens a different kettle of worms.
"The difference was not apprarent until my upgrade from ADSL1 to ADSL 2+ "
Can we have the full picture please? E.g. what speeds are you looking at? What wireless technology is in use on the various bits of kit (what chipset, what they are capable of, what they have negotiated to use).
If the speed difference is the difference between (say) 7Mbit and 10Mbit, you wouldn't have seen much difference anyway prior to the ADSL2+ upgrade because the DSL speed would have been the limiting factor.
It would be a bit of a surprise if the WiFi was now the limiting factor, but surprises do happen. A download from a local source will help confirm (or otherwise) this.