This is all part of the Quality and Outcomes Framework (QoF). Dementia is not the only disease/ailment that GPs get paid for. COPD, Diabetes, Depression to name but a few are all apart of this. Its an excellent idea as GP surgeries have to make targets. For example; if there are 300 patients who have Diabetes, then GPs have to get them in for their reviews. This makes sure that patients aren't neglected and keeps them falling ill and being a burden on the hospitals.
As cuntspotter rightly pointed out; any patient who may have dementia (there is a test that GPs carry out) will be reffered to psyche at hospital