http://www.miketheboilerman.com/Glow-Worm-Ultimate.htm

The Glow Worm Ultimate is a very good boiler in my personal opinion. Simple design, reasonably fuel-efficient, reliable, and easily mended if/when it goes wrong.

The Glow Worm Ultimate is a non-condensing boiler which was manufactured from about 1990 to 2003 when the regulations changed and condensing boilers because compulsory for new installations. Production naturally stopped as the market for non-condensing boilers vanished. It has a durable one-piece cast iron heat exchanger. (Most modern condensing boilers have single heat exchangers made from either cast aluminium or fabricated stainless steel.)

Ignition is electronic meaning there is no pilot light running 24/7 wasting gas, ans the flue gasses are expelled with a fan, so heat is not convected and wasted to outside during the boiler's 'off' periods.

Like the Potterton Neataheat, owners of Glow Worm Ultimates are increasingly being advised by younger gas technicians to replace them for no other reason than they are old. Often difficulty in obtaining parts is cited (which is a plain lie, parts are freely available form any heating spares merchant) 

All boilers have their individual problems and the Ultimate is no exception If you have a Glow Worm Ultimate, here are the common problems I encounter and the fixes. Most involve the pilot light!

 
Common faults:

1) Control board failure: 

The boiler refuses to light, often starting and stopping the fan and not progressing to ignition. A new control board fixes this. It's an unusually awkward and fiddly board to replace and takes half an hour of so, but the board is freely available as a spare part.

2) Blocked pilot light jet: 

The boiler sounds as though it is running but there is no flame. The quiet ticking of the spark can be heard but the pilot flame never lights. This is an unusual fault in boilers generally but common on the Glow Worm Ultimate. I have no idea why this happens but cleaning out the pilot jet with a thin piece of wire - a guitar E string is ideal - fixes the problem. Either that or fit a new pilot assembly. 

3) Main burners chattering on and off: 

This is also a pilot jet problem. As the pilot jet begins to block, the flame gets smaller but if it contiunes to light, a point arrives when the electronic flame detection circuit can just about, but not always, detect the presence of the gas flame. If detection of the gas flame fails while the boiler is running, is turns OFF the main burner, delivers a spark to re-light the pilot and then turns the main burner ON again, all in about half a second. It can do this repeatedly for hours on end. As above, the pilot light gas jet need cleaning out. I suspect any number of boilers have been scrapped due to this fault.

4) Ignition electrode erosion:

The electrode gets eroded by the spark and grows shorter as the boiler ages. This means the spark gap gets bigger. It remains small enough for the spark to jump and light the pilot flame, but the other function of the electrode is 'electronic flame detection', and the growing gap causes this function to fail first. The boiler lights the pilot but cannot detect the presence of the flame, so the sparking continues and the main burner never lights. A new electrode fixes this.