That's good to know, and I will buy a Clio Haynes manual for future reference. The plugs are apparently about £20 each - must be that special type you mention, but have only done 20K miles so good for a while yet. Your help is really appreciated - many thanks :-)

Many thanks for tips and encouragement! All done now, and have learned the following:

The plastic tray under the engine can be swivelled back and out of the way after removing the 5 bolts and detaching just one of the rear push fit connectors, making it easier to reconnect. The drain plug has a square recess of 8 mm, so I had to make a key - very luckily I had an old 8x8 mm bar at the bottom of a toolbox ! I would recommend make the key as short as possible, e.g. 2.5 cm. The filter appears to be impossible to remove without spillage, so wear nitrile gloves (cheap these days). The air filter (bought at the same time as the oil & filter) is fortunately a doddle to replace.

I agree the Haynes manuals are not what they used to be - soft covers, cheap paper, information jumbled up all over the place, the chapter and section numbering has gone mad, and there is no longer a troubleshooting section where you need it - at the end of each chapter. A very sad situation, as they used to be pretty good.