thanks again SoulFish. Took myself off to planetcatfish.com and they reckon 6.4" for multis and 5.2" for petricola, so there's not much in it. There is however a dwarf species of petricola which grows to 4", this might explain the confusing information.
petricola do seem to prefer a lower temp, 22-24, whereas the multis prefer 25-26, which is just about where I want it for the cichlids. Classic tang biotope guide actually says 27.5
one other key factor in this is that the LFS has multis but not petricola, let alone dwarf petricola. I also saw today a gold version of the multi (2 in the tank) but they were £50 a go!!
...and then finally when you think planetcatfish.com is your oracle, they list syno angelicus as 6.4" - yeah right! ;-)
so I'm thinking 3 multis in my 48 x 15, that's enough substrate for 'em and they can have a cave each even though they'll probably sleep on top of each other.......but will I be able to sleep tonight?