Melatonin will work IF you get the good stuff with a script from the doc, it costs about 50 fiat a month. The OTC stuff from the chemist probably won't do too much.
If you want a cheaper and easier solution, take phenergan (start with 10mg if that don't work move to 25 mg) take one before bed same time every night for a week, your body should be in rhythm by one week.
Get the phenergan from chemist OTC, if they ask say you have allergies. Phenergan/allersoothe generic won't affect any other common medications you maybe on, for example anti-depressants. If you feel a little foggy or groggy next morning and a cup of coffee doesn't improve your mental clarity, use a smaller dose of the phenergan.
Ps don't forget eating well, bit o'exercise etc will go a long way to improving sleep. Avoid booze too if u can.