Marigold.

tagetes....but couldn't swear to the variety

These are annuals. They are super easy to grow. You can buy seeds or many people have seeds they trade or give away. A marigold plant produces hundreds of seeds. There are all kinds and colors-mostly yellow, orange, red. They have a spicy smell that is supposed to repel mosquitoes. They bloom spring through frost. Kids often grow them. Just press the seed into the ground, make sure it's warm and water. It will sprout!

yas223 - in the UK you need to start Tagetes indoors. We don't get sufficient heat early enough to grow them from an outdoor sowing to flowering size. You will find seed in any seed rack and plantlets in any garden centre. You'll also find them in catalogues and online. For example, T & M has 34 varieties.They like heat and sun so often end up sodden in our summers particularly the large flowered ones. Look for Tagetes or African and French Marigolds. Calendula is also commonly called Marigold so make sure you get the right sort.

Here is a link that might be useful: One source

In the US these are known as marigolds. You can get seeds at any garden center, any major retailer - even your local hardware store - that sells plant seeds, or on line. Burpee and Park's seeds have some interesting hybrids including a white, green, and very tall and very double varieties.

They are the easiest things to grow and typically, school children learn to grow them as an early elementary school project just before Mother's Day. In the spring you can buy flats of them already in bloom but they are the easiest annual to start indoors. A little potting soil in an egg carton or a paper cup is really all you need.

These look like dwarf yellows, but there are literally dozens of varieties. They are a sturdy plant but do need ample sun and water. They are a natural insect repellant... I plant them all around my tomatoes and in and among the herbs in my kitchen garden.

Don't confuse them with marsh marigolds or members of the calendula family... these have a feathery leaf. Deadhead to encourage continuous growth and blooming all summer until the first frost.

The OP, yas223, is in the UK so I gave information specific to growing them here. Temps high enough to germinate them outside would not be reached until July/August. You need a greenhouse or light, bright indoor space and they wouldn't make an easy children's project. The plants can't be planted outside until late May.They are generally known as African or French Marigolds over here whereas the term 'Marigold' on its own generally indicates Calendula.

Are they available in UK in flats? My folks had a farm and raised these and many other annuals from seed beginning in later winter in the greenhouses. They sold flats of different varieties in the spring. Even here in US, you can get a number of varieties of these in flats in the spring. But they can be grown indoors any time if you have a greenhouse window or very bright window.

If I were OP and had a yard, I would order a packet of seeds after Christmas and plant indoors to put out in the yard, or purchase flats at the garden center if they are available there.

'...and plantlets in any garden centre...' So yes, you can buy them in 'trays' as we call flats.