In the majority of cases your new ISP is best placed to investigate and fix the underlying cause of the Tag (Ofcom has issued guidance to the industry which gives the new / gaining broadband supplier the authority to clear your line) and this process should normally be completed in 10 working days, but many are often fixed within 24 hours.
Tag's usually occur due to an error by the previous home owner (e.g. they didn't cancel their broadband service properly) or the property's old ISP (i.e. the service might have gone bankrupt before the customer could switch), which then fails to remove a marker (tag) that identifies a specific service as having control of the connection. If your new provider is unwilling to investigate the reasons for the tag and remove it then you may wish to consider an alternative ISP.
Ofcom's Tag on the Line Guide
http://ask.ofcom.org.uk/help/internet-service/tagremoval