sounds as though they may be a local company who have nicked the names of Homeserve and Heatteam (baxi ) to make Heatserve
all depends on how much they are asking for the cover? if its the same price as british gas ask then you may as well go with bg as you will get the best service.
you also need to be very careful to read the small print to see what you are actually getting i.e. bg have different levels of cover at different prices, basic cover would be the boiler only, then its boiler and the heating system, and there top cover would include boiler, system, plumbing and electrics etc
a cheaper option may be adding the boiler to your home insurance which may only cost arond �20 a year, but you will have to get someone in to service the boiler annually and f your boiler does breakdown then it will take longer to get it repaired.