Well I bit bit the bullet and had a go at sorting this out. I had to wait 5 days for my fingers to recover enough to use them to type again !
Overall, it went well but the first problem was that the offside (drivers) wiper arm couldn’t be persuaded to come off the spindle. In the end I just removed the passenger side scuttle panel cover and removed the old grommet, replacing it with the new part I’d received from MX-5 Parts. Relatively straightforward and I used the Fernox sealant on the metalwork before and after putting in the new grommet. Only niggle is I only heard one click but the new grommet seems secure so I may check it after the next downpour to be certain.
On the drivers side, I lifted the plastics and the grommet was in much better condition although looking up from the footwell the rusty streaks were evident. Plenty of Fernox LS-X around the existing grommet and hopefully that did the job. Obviously on both sides I cleaned everything up with Grimex Wipes before applying the sealant.
So, my final decision is whether or not to tidy up the internal rusty brackets.