Many many moons ago.... BNFL (and DSRL) had extremely rigorous (bureaucratic) regimes which dictated not just which 'i''s had to be dotted and which 't' s needed to be crossed, but also the font size, the colour and the colour density of the dots and crosses before anyone or any company was permitted on site. If your guy is needed for a specific task, then a short duration fully accompanied pass may be available. Without that, then there will be (were) all sorts of artificial hoops barriers and obstacles to be overcome by your company before anyone can even get on site. Including security clearance, evidence of competence (to their standards only, no equivalents recognised) and a fixed minimum ratio of h&s full time staff on site at all times your guy(s) are working... Which at the lunatic fringes used to require one full time dedicated h&s professional with each team of two window cleaners.... I developed a deep and abiding dislike of work in the industry because of the utterly inflexible jobsworth attitude of most (not all) of those I encountered. Somebody had clearly read "so far as is reasonably practicable" and decided "not having any of that on my site". Things may have changed. I hope they have. Your best bet is to ask your client sponsoring manager who will (should) be able to give you a list of all relevant requirements and restrictions. And explain if any can be relaxed to allow the work to commence before hell freezes over. Sorry, I've just found my Mr grumpy hat after missing it for about four years. (I've been retired four years)