vokera 12-48 mynute no relight after boiler thermostat clicks off?

hi can anyone please help. i just moved into a flat with a vokera 12-48 mynute standard boiler with power flue, running on a sealed system. the boilers fires up fine when turned on and cuts off when the radiator water temp reaches the boiler thermostat setting, as it should, but as the radiater water temp falls the thermostat clicks but the igniter sequence fails to initiate. when i turn the boiler off and on again (either switch) the burner sequence starts and the burner relights again.
water presure is fine, flame is good.
I am thinking along the lines of the igniter probably having 2 trigger wires, one to relight from thermostat and one on system start up but i can`t find a wiring diagram to confirm this.
boiler wiring looks to have been `played with` in the past.
any ideas?
many thanks....tim