My daughter had this same problem the other day, ran a bath OK but next time ran hot tap the boiler refused to fire up. She had not tried the heating yet only the DHW (Domestic Hot Water). The boiler is only about 18 months old.
Got the phone call "Help Dad !"
The installers had only left a user guide and a contact phone number ....
So the first thing to do was to download and printout the 22 page Manual from the Sabre website, as the User Guide has minimal information.
http://www.sabreboilers.co.uk/sabrehe.html Installation & Service Instructions (7.8mb).
Having looked up similar faults on this forum and studied the Manuals fault finding guides and ignition cycle process and interlocks, it was pretty obvious that the most common fault is a blocked venturi tube on these boilers, preventing the boiler to fire up. Another possibility was the condenser outlet .
Went round today and sure enough the boiler was making all the right noises - pump running and fan running but no ignition ! Looking at the boiler - at the top right hand corner is the pressure switch with two grey wires on it and the pressure sensing tubes, two clear plastic pipes going to the venturi and a straight vent pipe.
With everything switched off ! Disconnected the clear plastic tubes from the pressure switch and tried to blow through each one. The straight vent was clear, passing air, but the venturi was blocked, initially not passing any air through it at all, then clearing a bit allowing some air through. The venturi pipe cannot be taken off from the outside for cleaning (design error) unlike the other vent which is held by a single screw. So I used a length of multi-strand wire and stripped off about 15mm to form a chimney sweep brush and used this to clean the venturi tube blockage. The Venturi tube is an angled pipe so the wire had to flex around the corner then was pulled, pushed and twisted to clean the tube. I then blew through the venturi again and the air passed through with no problem, but note this tube offers more air resistance anyhow due to the right angle. Then put the tubes back on the pressure switch. I also did take a look at the condensing outlet at bottom right of the boiler to check it was also clear OK.
Then switched everything back on and BINGO - all fired up OK . A blocked venturi pipe as expected......although everything looked very clean on this nearly new Sabre 25HE somehow it had the venturi tube problem after only 18 months use.
"Thanks Dad, want a cup of tea and a chocolate digestive ?"