What is wrong with my Potterton Boiler Suprima 40 L?

I have the above boiler. I've had no problems with it for the last three years. I am putting my house up for rent and typically, it is now not working. It was working fine last week, I havent had the heating/hot water on for a week. popped back to the property to show some around and, no heating!

The process seems to follow like so:
- Solid Amber light when boiler off
- Switch on boiler amber/green flashing for approx. 30seconds
- red light flashing
- 60seconds to 1min later it tries to fire up again so we are back to amber/green flashing.
- then red flashing again.

I cannot locate a user manual for this model so so idea how to trouble shoot. Any ideas please?
- leave on