There is typically a fixed pay scale (see the UCU page, for example), but where on that scale someone starts may be subject to negotiation.
Within each job "band" (lecturer, senior lecturer) there is progression up the scale on a regular basis (usually, and usually one point per year). Typically people are promoted from one band to the next before reaching the top of the scale, but in principle you would just stop. Different institutions may place the boundaries of the job "band" in different places, but the number quoted in the advert usually corresponds to those edges.
On being promoted to full professor different rules apply, which seem to vary by institution.
There is no difference between fixed term and permanent positions.
Often a probation period will apply - there may be different rules in that time.
The salary scales are not accurate for all institutions - for example, London salaries are increased to account for the increased cost of living.