We got some rather good clematines from Tesco the other day, and have been
saving the pips.

we have a south facing window and hope to grow some plants from the seeds
and keep them inside.

is there any advice a complete novice should know about planting the seeds,
please? many thanks.

If you just want to see a plant grow, just do what you guess. Put them in a pot of seed compost, just below the surface, water them, put it on a warm windowsill, and some of them will grow. There is a good germination rate from most citrus seeds, and mostly they grow fairly rapidly.

If you want to get fruit like the one you just ate, you are wasting your time. I think the odds of actually getting the same plant as the one that grew your clementine have been estimated, for most kinds of citrus, at around 100:1. What is necessary to achieve this is that the seed doesn't actually germinate, but reproduces vegetatively as though it was a cutting. In this case, you often two shoots out of the same seed, and have to separate them and guess which one is the vegetative production. If you have the time, space and effort to plant a few hundred seeds, and spot the very few that produce two shoots, good on yer.

From a germinated seed, you'll get a plant. But it probably won't flower until it is tree-sized. If you get that far, you will probably even get fruit. But around 99% of the plants so produced will be "wildlings", ie, have nasty sharp inedible fruit. If you want sweet fruit off citrus, you have to propagate vegetatively.

You have better chance by growing a lemon, as sharp lemons isn't normally a problem. If you actually find a seed in a lime (which I haven't done for years), even better.

Your best chance of growing citrus fruit in England, especially if you don't have an indoor space for a 10-20 foot tree, is by buying a specially grown miniature plant from a plant centre. Even then, growing citrus fruit indoors in England is a time-consuming and difficult task, one on which I have experienced only failure.