Profile says Hilary is a lawyer, perhaps with little time for such stuff. But what we all have found is this: to hire somebody to fix a leaking port is expensive, but that's not it: it is that that somebody is hard to find, doesn't show up, and has little interest in small jobs. So most of us shrug and do it. It's kind of a given with an older sailboat. You come to own tools and skills and with them a fuller appreciation of tradition.
In the case of fixing your leaking portlight, instead of replacing it, there are probably three causes of water entry, listed in probability.
1. The caulking between the frame and the cabin house has failed. Looks like your frame is secured with through-bolts and castle nuts on the inside. Remove the bolts and the frame will separate. Urge it out of its hole by prying with flat objects. Clean up the old caulk by scraping and if necessary caulk remover. Buy a tube of with Boatlife Life-Calk and rebed and reattach the frame.
It's usually possible to confirm the leak is caulk failure by a close look at water entry. In doubt, direct a hose on the port and note the ingress.
2. The gasket on which the lens presses, when dogged closed. Such gaskets are usually replaceable.
3. The union of lens to frame. In some portlights, the lens can be removed from the frame and reglazed. In others, no dice--the design doesn;t permit removal and the whole portlight unit is discarded and replaced.
This forum has many threads on such jobs, since everybody has to do them. I don't know why, but repairing a single portlight suddenly makes the whole sailing thing make sense, improves hand on the tiller, brings new business opportunities, and makes you sleep like a baby in the cradle of justification.