We regularly see situations where a search project has been set up as ‘fire and forget’ – which is never a good idea: not only does content grow, but user needs change and search requirements evolve, whatever the application. Search should be a living project: monitoring user behaviour should reveal not just which searches ‘work’ (i.e. the user gets some results which they then click on) but more important which ones don’t. For example, common mispellings or acronyms might be a useful addition to a synonym list; if average search response times are lengthening then it might be time to consider performance tuning or even scaling out; the constant use of the ‘Next 10 Results’ button might indicate a problem with relevance ranking.
Luckily any improvements to gov.uk made by the GDS team should appear in their Github repository at some point – as I mentioned before the GDS team are (very sensibly) committed to an open source approach.