Sleep problems can be primary, or they may be a symptom of psychological or physiological problems.
Primary sleep disorders are a bear to deal with. Unfortunately, your solution is not a good one as alcohol interferes with normal sleep patterns. What you need is a good workup that looks at your history of sleeping problems, any physiological problems that may interfere with sleep, and any psychological problems that may interfere with sleep. In other words, keep trying to see a doctor. I bet you would be a good candidate (assuming it is a primary sleep disorder) for a medication that regulates sleep while allowing for a normal sleep cycle. In the US, a low continuous dose of Elavil (amitryptilene) would be the answer, but I don’t know if it is use in the UK. It is an anti-depressant that at low (not the higher doses used for depression) is effective for sleep. This is different from a sleep medication that acts by making you sleepy and is only for occasional use, like Ambien (a gift from the gods) or Sonata.
In the meantime, I second the suggestion that you try some of the behavioral techniques. The most frequent ones I have heard (I am a former insomniac myself) are to limit caffeiene, go to bed and get up at the same time every day, and use the bed for sleeping only. That last one makes me laugh, because when you read it, they mention not working in bed, but sex is ignored. On the other hand, the couch can be nice.
One thing I noticed is that you do shift work. Does your shift change? Do you tend to change your hours on your days off? For some of us with sleep problems, that sort of stuff is just impossible.