Mike - I suspect it is because if not there would be flooding everywhere outside with the water seeping out of the manhole cover. Will check it again tomorrow morning in the light but I'd be surprised if that were the problem.
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.
Are you a trade or supplier? You can create your listing free at DIYnot Local