I had a only-slightly-similar situation that I had an original narrow skylight in a Victorian roof between two rafters at (iirc) 400 or 450 centres with a.lath& plaster light well coming down between the ceiling joists at wider ctrs.

I wanted to replace the window but couldn't find a velux style product that was narrow enough to fit.

In the end a joiner made up a new fixed hardwood frame to suit the opening and a local glazing guy (his son!) made up a glazing unit to fit in it, and dressed it with flashings etc. I insulated the light well to match the ceiling.

(Edit: sealing and painting the inside of the light well required some contortions while standing on a stepladder, helps to be skinny!)

It looked fine, before and after!. You don't actually need a big window to bring light in, skylights get more than wall windows do.

Think about whether you need to clean it and if so how.

You probably already looked at those sun tube things? Not bad.

Or yours will probably be ok to cut away parts of one or two trusses to form a bigger opening IF the edges are detailed to carry the loads into the neighbouring trusses which might need to be doubled in that area. Like how a loft hatch is formed. Would be best to ask engineer to detail this and the hole might have to avoid certain points of the truss such as near the eaves or any load bearing internal walls.

Thanks Tom thanks Will; I hadn't thought of any subsequent cleaning access on such a narrow, 400mm, gap; good point. I had considered Toms idea of crossing one or more trusses and leaving the trusses exposed allbeit smartened up. That approach with a relatively narrow rectangular, landscape fixed light across say 3 x truss gaps is maybe the simplest way forward. " Jolly colour " I like, which one may be out of my hands though and I see 40 quids worth of sample pots on the way. Keeping the rectangle narrow and horizontal has the added advantage of positioning it to best avoid the structural "gang nail plates" used in truss construction which wouldn't look good exposed. Also it'd allow better external placing to match tile layout.

Thanks tony, good to know you too think is a decent plan. I just have to work out how high the light should be back to the room below bearing in mind the ugly gang nail plates, otherwise I may be into "wrapping" the exposed trusses with a thin timber skin, e.g. something like 5mm Oak or clear softwood, technically not difficult but time consuming BTW it's N. facing.

If the timber trusses are painted with an (any) opaque colour then the nail plates will disappear within the uniform colour