It seems to be fairly standard procedure for ISPs to blame BT, for BT to blame the ISPs and for both to blame your PC!
If both you and your wife connect by ethernet cable to the router, even if one PC is switched off, try disconnecting its cable to see if the speed is still slow on the other computer. Then repeat the procedure with the other computer physically disconnected from the network. If the results still appear to be slow, see what results the speedtest.net site shows.
If you're connecting both computers wirelessly, see what sort of download speeds you get with a wired ethernet connection.