Vueling still owe me compensation after 4 years! HELP!

AZW

I realise that if you read this you'll probably think I'm a muppet but there is a reason why I haven't been able to deal with this ongoing issue (COVID, dying parent, sick nephew..you get the picture).
In July 2018 we were having a family reunion with people travelling from all over the world to Italy, where my mum came from. We booked flights with Vueling to Florence. My sister and nephew booked separately to me and my daughter but on the same flight. 10 mins before departure it was cancelled. I tried to get Vueling to find an alternative flight but they refused. Instead they offered us flights to Rome - 3 and 5 days later, with no suggestions about how we were to get from Rome to Florence, costs of additional travel or anything about the fact that the reunion would be over by then!. So instead I spent hours on the internet trying to find an alternative. I could have booked flights with BA but they were £1000 each but instead I managed to find a route from London-Paris, Paris-Florence getting in 24 hours after the intended arrival time but in time for the reunion. It cost £2800. I wrote to Vueling asking them to pay for the cost of re-routing plus the legal compensation required under EU law. They refused. I contacted the CAA who confirmed in writing that vueling were required to pay up - they refused. I emailed and emailed and they kept refusing. Then I tried using EECNET and they were great at getting Vuelings to speak to them but still refused. Then Vuelings told ECC that I had already complained to the CAA and ECC said I had to get the CAA to finalise matters. Back to the CAA who contacted Vuelings who told them they would pay me - letter from CAA confirming so they were closing their file - Vueling refused to pay!! I have been trying to find out if I can sue them but then my mum got very sick, and then covid and this seemed the least of my worries. But now I want to get it sorted. So if anyone knows how I can sue Vuelings in the UK - perhaps because they are owned by IAG? or some other way to get them to pay up I would be very grateful!