[robert mitchell]

If you fit three plastic blocks inside the top of the lid to the sides using long reach bolts leaving sufficient room to add large thick washers and nylon nuts to the end of the thread to keep the blocks from falling into the tank.   Roughen both edges with coarse grit, clean with alcohol rubs, then apply tiger seal. Close up the gap by tightening the nylon washers down so the two sides butt up tightly and leave the sealant to dry for at least two hours the tank should be secure and strong enough to last for a long time.

Not a bad idea that , possibly even work with decent self tapping screws .

[Perfect Windows]

Have emailed a couple of places .......... im toying with the idea of self tapping some angle iron over it with silicone underneath .......good idea?

I'm late to the thread but if you can take the strain off the joint you can seal it. If you use CT1 sealant (you can get it from good plumbers merchants - https://www.ct1.com/our-products/ct1/ ) you don't even have to drain the tank or dry it out. I sealed the inside of a cracked cistern with it about three years ago while it was full of water and never a drop has leaked. Mate of mine who is a plumber recommended it and it's brilliant. The only problem is that it won't hold the strain so you need structural support.

Vin