More than likely going to be something at the Fast4 end to be honest. Try the Heart Internet dial-up service on 0845 604 0489 (use any username and password of your choosing) and see what speed you connect at.
Alternatively, give the UK2 dial-up service a try on 0845 665 3000 (uk2 is both the username and password).
Bear in mind that both of the above are the cost of a local rate call per minute!
If the speed increases with either of these it confirms a change has been made at the Fast4 end of the connection.