Advice on a horse that refuses to go forwards please?

I hack my mare regularly, sometimes with another mare, and sometimes with my Mum's gelding. Here's the problem; she's great with the gelding but nappy with the other mare. 18 months ago she used to spin at anything she didn't like or was scared of. Fortunately this has gone only to be replaced by standing stock still and refusing to budge. I've tried softly softly approach and 'not so soft' to get her to move on but her tactic now is to reverse then stand still. as soon as my leg goes on, she reverses again. Trouble is, if I get off, she won't even be lead by me. Also, as soon as she slams on the anchors, the other mare refuses to go on also and employs the same tactic. so here we are with two reversing horses and two frustrated riders. Only solution is for my friend to dismount and lead as her horse doesn't seem to lose the plot quite as much as mine. We are very experienced riders and do a lot of hacking, trailer, beach, roads, off road etc etc.t his behaviour significantly has started since the heavy snowfall we've had over Xmas. Any practical solutions gratefully appreciated please except for comments like, 'Only ride with the gelding' or 'Go back to groundwork and have lessons'. Riders with similar experiences will know where I'm coming from!