In the course of doing this work I noticed a slight blow from the exhaust where the manifold meets the downpipe. Another delay whilst I ordered the parts then even more delay when I found I couldn't undo the nuts, so I had to farm the job out.
I finally got the car back on Monday and I can't believe the difference. It's like driving another car. It's quicker off the line, has much better throttle response once moving and cruises with a lighter throttle opening. Much quieter too. With changing the air filters as well as doing the valves, I can't pinpoint what has made the most difference but I suspect it will be the valves. I wonder if it will do anything for fuel consumption!