In my experience, gaps do fill in, though slowly, from the sides and from below. A light feeding might speed up the process, though heavy feeding will cause it to lose its shape.

Yes agree, in these situations you can bring some of those longer sides round to the bare patches and weave them in, then gently wire them in without damaging the stems by tying to tight to main stem, use some padding eg leather to put round main stem and tie in loosely.

Thanks. I'll try the weaving. They are in big pots so I'll turn the bare patch against the wall. I try and rotate the pot every couple of months but luckily only had the 1 short freeze spell.