Can only fit 16mm back boxes

[wannocks]

My walls are made from hollow concrete blocks with a wall thickness of around 15mm. All the original sockets are single gang in 16mm boxes surface mounted on the block. I would like to change these to doubles.
Is there a way around chasing deeper for double boxes, or are converta sockets the best way to do this.
The other problem is that the cables are surface mount with plastic capping. The plaster is a very thin skim over this and is cracking in several places. Would it stand up better if it was replaced with metal capping and re plastered?
Any advice would be appreciated.

 

[flameport]

If the walls are hollow, don't even think about chasing there as you will just create huge holes.

The converter sockets will be the quickest and easiest method, BUT only if they actually fit onto a 16mm box - seems unlikely. If not, then surface mounted sockets would be an option.

How old are the existing sockets and wiring?

 

[ban-all-sheds]

Dry line the walls - that'll solve both your problems.

 

[wannocks]

Thanks guys, I think I'll take both options! One wall leads into the stairwell so I don't want to dry line there as it will be a massive job to get around the stair stringer and then up 2 floors (I don't want to have a step in the wall.) Might dry line the worst offending couple of areas and converta socket the others, I've tried one and they do fit. Bloody 70's construction methods