There have been several threads about train planning in the past (it may pay to make a
Google search using the limiter "site:railforums.co.uk" as the forum search function isn't great).
But in brief - long term planners work on the base, i.e. LTP, schedules which then get amended as required on a weekly basis by the short term planners.
In principle these schedules only change twice a year, in May and December - but even during "normal" times, timetables do sometimes change part way through these periods to accommodate long term engineering works, stations opening/closing, leaf fall alterations, summer extras etc. And in the last 2.5 years most TOCs have done something like 10-20 timetable changes (some even more than that), so clearly the workload has been much higher than usual.
It's also worth noting that there are several stages to implementing each biannual timetable, from initial conception through various working stages, a bid, an offer, and then any post-offer changes. These processes stretch over a period of many months or even years, and so you will often be working in parallel on a May and a December timetable (possibly even further ahead than that for a total rewrite, as is currently 'on hold' for the ECML).
At various times, LTP planners may also assist STP planners to cover for holiday/sickness or a particularly busy week/period. They may also devise a timetable that the STP planners can bid, if there's a particularly disruptive piece of engineering work that will be repeating for several weeks. And there is also conceptual work, which explores the possibility of changes to future timetables based on "what if" questions the DfT/funding agency might pose.
Each TOC does things slightly differently in terms of the roles that timetable planners have - whether people are multi-skilled and write traincrew diagrams as well as creating and revising schedules, or whether they're just specialised in one discipline. There are also differences in the way that e.g. large networks are split up (e.g. regional vs long distance, west vs east).
It's not the best paid career in the world and obviously current pay freeze imposed by the government is not helping with that, but it's one of those 'niches' where if you enjoy it, you could easily spend your whole career there.