I'm also on the Purleigh exchange but more than 500m from the exchange and consistently sync at 24Mb(down)/1.27Mb(up). Since the Purleigh exchange was upgraded to ADSL2+ in April 2018, I've NEVER seen any sign of exchange congestion, and achieve >20Mbps downloads at any time of day.
It was certainly the case when the exchange only had 8Mb "ADSL Max" that many lines suffered congestion, and that the DLM algorithm in the old equipment caused ever increasing SNR margins on low error-rate lines - to that point that sync would steadily slow over time !.
My advice would be to upgrade to FTTC, currently £23.99pm with Plusnet for unlimited 40/10 including line rental.
You might find it informative to go to the presentation by "County Broadband" at 19:30 on Monday 11th March 2019 in Purleigh Village Hall, for a discussion on them building a new 1Gb fibre to the home service in our village.