should be easy – but its annoying me. Two brackets are attached to the wall, these line up with ‘pockets’ on the underside of the sink. So in theory you work out the position of the brackets, fix them to the wall and then the sink slots on top and is stable and level.

Here a pic of how it is installed (not the basin I have)

I’ve done the above, brackets line up with ‘pockets’, brackets are level but the sink is not stable. It stays on the brackets but it tips forwards (the top of the basin in the corner comes away from the wall)…why would it do this??????

i can solve it by packing the corner where yhe base of the sink sits but this should not be needed

Not a straight wall or corner.
Pack it, seal it.

Is there something to fix it to the brackets like some bolts?

It might only become fully stable with taps and pipes to triangulate it.

Nothing to fix it to the brackets, but there are 2 holes closer to the front of each side in which I can fit a stabilising screw. So my plan at present is to fit as is, stabilise with 2 front screws, pack the rear (!) and put a load of silicone between basin and wall. We have 2 young boys so it’s stability will be tested!

Walls seem pretty vertical and corner is pretty much 90degs so it confuses me.

cover the whole of the bits that will be in contact with the wall with silicone sealant.

will ensure it never leaks and also act as ‘padding’ to stabilise it.

be careful putting screws through the holes – use big washers with rubber washers behind them as the sink won;’t be good if you put any twisting force through it and may crack.

Yep, washers were planned for the screws and a load of sealant to help glue it to the tiles behind.

It might only become fully stable with taps and pipes to triangulate it.

This
Plus silicone all round before and after tiling to seal/bound

the brackets are simply locators rather than a fixing point. we normally use the bracket for initial support and silicon the thing within an inch of its life then remove the brackets as they are often visible especially when sat on the loo.

..

either shim out the bottom of the brackets or bend the brackets so the fit is a bit tighter?

The porcelain will not be perfectly square.

With three boys in the house, are you also having a home urinal with an aiming fly?