Swap the ignitor from left to right to see if the problem is the ignitor. If not the ignitor, swap the bulbs to check that. Finally swap the ballasts, which are bolted beneath the units.
You do need to be a tiny handed ambidextrous contortionist with eyes on long stalks to do all this, but at around £100 for bulbs and ignitors, and £250ish for the ballasts you want to find the problem before you spend any money. Mind you, if you have a swear box with you, you'll soon have enough!
I bought an ignitor off Ebay, but while I was waiting for it to arrive i left the cover off the back of the h/lamp which allowed water to get into the ballast! Luckily for me I was able to dry it out before it caused any damage.
If you're anywhere near Southampton, I'll stand by with a drink giving precise instructions while you do it.
I have no skin left on my knuckles!
TonyZ