Does rigid foam insulation break down over time?

Don't expose rigid foam directly to sunlight for extended periods - UV will break it down faster. Once it's covered with plasterboard inside, you're pretty much fine for 20-30 years and beyond. XPS and EPS are stable when they're protected from light and moisture, which your internal installation handles. The bigger concern with retrofitting an 80s bungalow is making sure you've got proper vapor barriers so moisture doesn't get trapped and cause problems down the line.

Is your foam going to be behind the plasterboard the whole time? Then you're golden - UV isn't really a concern once it's covered. Both XPS and EPS are pretty stable indoors, and they should easily last 30+ years without much degradation. The main thing that actually matters is moisture exposure and physical damage, not time passing by itself.

The good news is that rigid foam's durability isn't really the concern here - it's genuinely stable stuff once it's tucked behind your plasterboard! UV exposure is definitely the killer (your surveyor's right about that), but when it's covered indoors, you're looking at basically no degradation over 30 years and way beyond. XPS and EPS both hold up brilliantly in protected applications, so you can confidently retrofit with either one knowing it'll outlast most of the rest of your house! The real maintenance thing to watch for is moisture getting trapped in the foam layer itself, but proper ventilation details in your retrofit design handles that pretty easily.

I've dealt with plenty of insulation choices over the years, and the one thing I'd push back on a bit is the "golden" comment - rigid foam absolutely can degrade over decades, but it's a slow process that matters way less when it's covered than most people think. Your surveyor's right about UV being the culprit, but once that plasterboard's up, you're looking at minimal breakdown over 20-30 years; the real issues with foam tend to be moisture and poor installation rather than time itself doing damage.