It looks like the damage may be being caused by Hebe leaf miner, when i have problems with this pest i cut the affected parts away and spray with provado bugclear and the results are always good.

I am tempted to say that they are badly viroused plants and would best be destroyed. But wait until you get other responses, Rubysmum.

Have had the identical problem with a parent Hebe purchased last autumn plus a dozen cuttings taken from it and kept in greenhouse, disappointed.

If the problem is repeated in plants propagated by vegetative means, cuttings, then that indicates a genetic problem with the parent plant, Ian.