I did an oil and air filter change a couple of days ago and had to remove the battery to get the air filter out. The car was driving beautifully before but now there are some issues with power loss and lack of response to the accelerator pedal.

The power loss tends to only happen at higher speeds/revs. So I may going up a hill quite happily at 70ish (~4000rpm) then, embarrassingly, I'll suddenly loose 5mph and ~300rpm. It idles happily at 1000rpm but, now I come to think of it, it does occasionally over rev at start up.

A similar thing was happening last year after I had to leave the car unused for a few months but, after a few days or weeks, it disappeared.

I don't really know enough so this is a stab in the dark... the cars electrics are not perfect, could it be that something is up with a sensor somewhere and the ECU needs to 'relearn' the driver input->sensor value(s)->engine power output mappings (if that makes sense) because the battery has been removed?

Have you had the clutch looked it . Could be a slipping clutch ?

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Not had anyone look at the clutch but had it a new clutch kit put in about 25,000 miles ago so I can hopefully rule that out. I did do a little test where I put it in 5th gear going ~15mph and it both wanted to stall and could be recovered by upping the revs so I assume it's not slipping?

Have you had the clutch looked it . Could be a slipping clutch ?

then, embarrassingly, I'll suddenly loose 5mph and ~300rpm.

I don't know much about clutches but surely if the clutch was slipping the RPM would go up, not down (or it wouldn't change but you would lose speed). So hopefully it's not the clutch.