Sorry been away so just now catching up with all the responses to my original post.

I think someone asked what pressures I'm runnning at. On the sides of my exisiting Maxxis Ardent Race Exo tr 29x2.20 tyres it says between 35-60psi and I normally run them at between 50-55psi on all surface types. I have tried at max 60psi and there is a slight improvement on tarmac and road surfaces but bike still feels sluggish. So I still want to change the tyres to ones less knobbly... especially as I hardly ever do any sort of trail and only ride on tarmac, gravel and road.

I've had a look at some of the tyres that have been recommended but noticed that none went up to the same width as my existing ones i.e. 2.2" or 56-622. I do like how my 2.2" tyres look on my bike and ideally would like to replace with 2.2"semi slicks.
A few of the tyre recommended went up to 2.1" but most seem to be max 42-622 or 47-622 ie below 2" in width... but 2.1" being only a few mill smaller than my existing 2.2" I guess would look about the same on the bike?