Sound like you could have Gravity Hot Water system Bax 572 is Very Old
There may be a non-return valve in the Heating Flow pipe near o the Boiler or pump which is letting by,
I don't think that this problem is due to the chap that serviced the boiler, but it would be wise to contact him (assuming he is correctly Registered) to investigate, as he will have more knowlege of the layout of your particular system.
Boiler Repairman - Ipswich, April 2009