"My local roofer, who I need to get the work done anyway, was surprised when I asked them to get rid of the moss, since they always believe that it helps to seal, and to bind in some cases, which I must say was always my view too."
Get a new roofer. A moss-covered roof is a ticking time bomb.
Far from having any beneficial effect, moss will ultimately lead to damaged and/or missing tiles. Moss is most commonly found on sand-faced tiles, or old, handmade 'Tudor' type tiles that have a slightly rough surface. The tiny irregularities on these tiles provide the moss with a perfect substrate on which to grow.
Once moss is established on a roof it will hold water and atmospheric detritus, and in cold weather this will freeze. Repeated freezing and thawing cycles will lead to the breakdown of the tile surface (called spalling) and ultimately the failure of the tile - it will crack and leak, or part of it will fall away completely.
There are various compounds that can be applied to a roof to kill moss, but you'll have to repeat the exercise at frequent intervals. A common way to treat moss on a roof is to spray it with a copper sulphate solution. A better,longer lasting, but more drastic and expensive solution is to remove the ridge tiles and re-bed them with a thin strip of copper ribbon bedded along the bottom edges, projecting about 20mm from the tile edge. Rainwater will wash across the projecting strip and over time will treat the whole roof automatically.
if you go for the spray method make sure your roofer comes back after a while to clear all the dead moss that will accumulate in your gutters.
Whatever you decide to do, get rid of the moss completely, and don't let anyone tell you it is in any way good for a roof.