Well funnily enough in that same area there is a very large council owned space that's had various uses over the years, from once being a huge deckchair storage area, to a council run woodworking shop - the door to get in, is literally right next to the Hole In The Wall and the owner told me he'd been in once and it's huge - he also suspected that there were other spaces nearby and had heard rumours of tunnels etc. The janitor story checks out too, I was told that a tunnel led (or at least did at one time) from The Imperial Hotel to these spaces as an emergency exit for visiting dignitaries such as the political party conferences, should an attack take place similar to the Brighton bombing - that way the vip's could be evacuated safely to cars driven down onto middle walk and from there to safety