The carpet has indeed now gone lol.
Since watching the edition of 'in the factory' regarding mattress manufacturing on the BBC, I realise just how old fashioned in design the sprung bases are! However there are no funds for replacement so they will certainly stay for now. Having said that I think if I were going to change the covers at any point in future I might consider adding a small memory foam type 'topper' to the mix, under the new covers (which would have to be tailored accordingly to allow for the new height).
The original reason for doing this job was merely to try to get rid of the horrible smell that was present. After some weeks since doing the work I'm delighted to report that it was a great success in that respect. The smell inside is now really nice and fresh. I'm so glad I made the effort.