Callan1:
The first thing to do, if you haven't done so recently, is to obtain your £2 statutory credit report and check it for completeness and accuracy.
Ensuring the electoral roll and residency data is complete is the single biggest issue but if already correct no improvement there. Searches drop off after a year so an obvious area for automatic improvement but perhaps not in 3 months.
Other accurate adverse issues unfortunately remain for 6 years after settlement so probably no quick prospect of improvement.
Perhaps you need some realism. Credit is not guaranteed and an often overlooked issue with credit ratings is that earnings information is not stored, only debt and available credit. A lender on the other hand, also considers affordability using earnings information provided on the application form. So if you have recently extended your mortgage your CRA credit rating may not have changed but affordability has taken a hit. There is no honest way to improve that unfortunately.