Dental resin or similar

Do you really mean hard. or strong? I'll assume strong, which is what you want for mending glasses.
Dental resins are usually bisphenol methacrylates with a lot of filler. They are designed for teeth, and aren't really that suitable for other uses.
What I think you want is a structural methacrylate / polyurethane adhesive, or else a really good epoxy. The structural glues can be (meth)acrylates, or urethanes or quite often both at once. They'll usually have -acrylate in the name somewhere though.
Typing "structural adhesive" into ebay gives several options. Note however that you need fresh glue - for these glues the use-by date can really mean exactly that. Keeping them in the bottom drawer of the fridge can extend their shelf life.
Or, quite possibly a good epoxy is what you need. Not all epoxies are the same! There is a huge difference between a fresh first-class epoxy and cheap araldite in tubes which has been kicking around for years.
Epoxies are also a darn sight easier to use than the structural acrylates/urethanes, which can have very short setting time, which necessitate the use of mixing tubes which mix and dispense the resin all at once, and which have to be thrown out after each use.
I (used to) use NHP 30 minute epoxy glue for mending-spectacles type stuff, but I don't think it's available any more - however I'm told Devcon and X-poxy are about the same. The 30 minute versions are considerably stronger than the 5 minute versions, which I'd avoid.
I use West systems epoxies for non-gluing purposes.
I personally wouldn't bother with the structural urethanes/methacrylates unless I need to glue very demanding stuff - I have some, but very seldom use them.
Note that not all cyanoacrylates (superglues) are the same either. Cheap ones can be used to tack things together temporarily for later seperation, but if you use a good one, properly, on eg steel you will not be able to mechanically seperate the joint. Ever. The steel will break first.
HTH,
-- Peter Fairbrother

Thanks for that comprehensive answer Peter!
I'll look in to the epoxies you mention.
Dave

I've just been asking round a few rocket people, who used to use the no-longer-available NHP expoxy, and the general conclusion is that Devcon Two Ton is a more than acceptable substitute.
HTH,
-- Peter F

I used to use Super Steel Epoxy. Don't know if it's still available, but used it for things like gluing rear view mirrors back on to the screen in the old Audi 100. Never fell off again and the stuff sets hard enough to work after a few minutes, then rock hard the next day. Quite a few of the b&q offerings, araldite and devcon included were rubbish in comparison.
Specialist (aerospace etc) companies like Silmid in Birmingham have a vast range of adhesives for just about every application. Worth using their site as a catalog to see what's available...
Regards,
Chris

Dave A drukte met precisie uit : Could Bison Metal EP glue help?

Super Steel sounds similar to JB Weld which is black so may have limited appeal.
Henry