The lender should have used form DS1 to notify the Land Registry that the mortgage has been discharged. So I advise hassling the lender to do so. (When I tried to sell part of my garden to a neighbour I found that the Land Registry's records still showed that there was a charge against the property in favour of the The Woolwich, despite the fact that I'd paid off the mortgage over a decade earlier. I contacted Barclays Bank, who'd taken over The Woolwich, to get them to lift the charge). It shouldn't cost you anything to get the charge lifted by the lender.
Otherwise you'll need to complete form DS2 and send it to the Land Registry, accompanied by the appropriate evidence of identity and the Scale 2 fee:
https://www.gov.uk/guidance/hm-land-registry-registration-services-fees#scale-2-fees
If in doubt, contact the Land Registry on 0300 006 0411. (0830 to 1700, Mon to Fri. Calls are charged at the same rate as those to 01 or 02 numbers, from both landlines and mobiles. In many cases that will mean that you won't have to pay for the call, as it will count as part of the 'included minutes' in your phone deal).