GREENSTAR 28i junior\iflo digistat scr\drayton rf601 not keeping temp?

I have a greenstar 28i junior boiler connected to a iflo digistat scr which connects via wireless to remote drayton rf 601 thermostat.
The boiler has a built in mt10 timer which wont work in timer mode so I have put it on the on position and control it via thermostat.
Recently finding that although I leave it on 18 C during the day and increase to 21C in the evening and before going to bed I reduce it to
18 C. When I get up in the morning the temperature has dropped to say 16C and the boiler no longer stays on and not maintaining 18C
Find I have to switch off boiler and isolating switch and then switch back on for it to work again.
Appreciate help on where fault may lie.- is the mt10 going to a off state even though it is on the on all the time position as wont work on timer mode.
When this occurs the digistat has green led (position 2)override) lit.

Also not happy with the MT10 so will the dt20 be a direct replacement for the mt10 and will it still work with my drayton rf601
Is the fault with the boiler or controls ?
Many thanks and appreciate any help with this.

Phil
November 2012