Hello Neil, any overheating issues? You might use a C3 - extension leads used because of not enough socket-outlets (553.1.7). Is the socket-outlet installed high enough (553.1.6) to prevent the lead from getting damaged? (522.6.1)

If the 4 way blocks are a design intended to be screwed to a surface (some are only designed to be free) and are fed through a 13A or lesser fuse, with cable that suits the socket design, then really there is nothing to object to.  Considering the mechanics, it is probably less likely to be damaged than a free extension lead, and electrically very similar.

Done well there is no reason it should be none-compliant. It does sound a bit cheap and cheerful, and may not be good practice  in a kitchen or other place with many heavy loads, and as above it would be sensible to inspect for overload, and maybe a shorter than maximum  re-inspection interval is prudent.

Mike.

(Not sure if I'm missing some earlier replies)

Electrically a FCU is pretty much equivalent to a socket and plug, so not much different to a socket supplying a trailing 4-way, or indeed a fused spur feeding a couple of doubles, so I don't see any great objection in principle. Presumably the trailing sockets are of the type with a facility for wall mounting (rather than having been drilled on-site which might undermine some protective measures). If it's supplying lots of small loads, overloading is unlikely to be a problem. If the small loads have high earth leakage (protective conductor current) there might be a concern about c.p.c. integrity, but otherwise for me it falls into the unconventional rather than wrong category.

   - Andy.