Circadin is 2mg slow release. I have tried over-the-internet melatonin before and it always made me feel terrible the next day - a total zombie. Circadin (or the generic prescription form) has never had that effect on me (which makes me question what on earth is in the over-the-internet stuff). If it does turn out that you need Melatonin I would really recommend trying to get a prescription for it rather than taking the stuff you can buy over the web, because how you feel the next day really is completely different. The only problem is GPs don't like to prescribe it and you generally need a private prescription for it.
You can buy all sorts of high dosage in the over-the-internet stuff but I've never needed it. I had my melatonin levels tested a while back and they were quite high so Circadin is clearly working.
A quick Google tells me that Bisoprolol does have a lower risk of insomnia than propranolol but it is still a known side effect of it, so if you're taking it in the evening it would seem possible that it could still be affecting you. Definitely worth speaking to your doctor about it. Good luck! xx