Hi there - hoping for some advice

I was checking my Experian credit report in preparation for possibly remortgaging and was shocked to see that my report had gone from Excellent to Poor. I went through my report to see what the issue was and there has been an Account in Default added by Lowell Portfolio. I have no idea what this is and all the info I have to go on is -

Opened 19th Dec 2008
Default date 23 Mar 2020
Satisfaction Date 27 Apr 2020
Last Updated 07 Jun 2020

It is also showing a 0 balance.

I have since checked on Clear Score and my score has gone up in that same period with them but again I dug around to see if anything had been added and yes there had been a closed account added by Lowell in July but it was not showing as default, just closed with a zero balance.

I have since run a few soft searches for credit card transfers etc and all that is showing now is credit cards for poor credit and high-interest loans etc.

The last few years have been a long slog to clear debt and get my credit report up to scratch and I'm just gutted by this. Any advice would be much appreciated.