I am using 9.20.
I will carry on trying and will let you know how I get on.
Many, many thanks for your help and patience :)
The sample SVG you sent certainly causes inkscape to struggle on my machine. It's probably the heaviest SVG file I've seen to date. Filters are certainly the issue you are having. Whilst editing the file, you probably want to turn these off using:
View -> Display Mode -> No Filters.
Obviously you're losing a lot of detail there when editing like this, but the program is very fast with your file in this mode.