is the upstand all the way round? I have fitted trays with upstands into walls only, that are tiles over later. I would recommend you follow the manufacturers instructions, sealing on the outside possibly, but (usually) never on the inside.
The size I am buying has 3 or 4 upstands although as you say I would prefer just 2 as it is going in a corner. With 3 or 4 the door and end panel would sit inside the upstand which would make it hard to seal but is it necessary
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local