The real reason they had to pour concrete in:
The Lurker Beneath was stirring: the stars were right and certain more obscure conditions met as well. The Lurkers were heavily warded and festooned with alarms and charms - nothing in ancient, wicked London stirs without people knowing about it. But this time the Thing Under Victoria ("Old Albert", wags called it) was moving far beyond expected parameters.
There was nothing for it but to deploy the concrete casement - hundreds of tons of quick-mix quick-setting cement installed during refurbishment back in the 1970s - and hope that the Thing would be trapped before it could really wake up.
Sensitive Londoners would suffer smothering nightmares for a few weeks, until spring lifted the psychic pall. For Bob Howard, the paperwork would last much, much longer.