no its not to late but if they are first year plants they may not flower until next year anyway. but they need planting now any way.

Thankyou SBG.

SBG, I tried to plant them today but the clay under my little meadow patch is too solid to even dig with a trowel. So I've given the ground plenty of water and left it to soak in. I'll give it some more later on. Do you think I should add some grit when I plant them tomorrow or will they cope with the clay as it is? I thought about adding some garden compost too but then thought it might be too rich for wild flowers.

I would mix a little composted sawdust or coir in, to keep the soil from turning back into a brick immediately after planting, just to transition the plants to the native clay from the plug compost. The main issue that wildflowers have with "rich soil" is the higher nitrogen content, which isn't an issue with sawdust or coir. Peat might work, too, but it will reduce the pH, which isn't often a good thing.
Also, let the clay dry a bit so it crumbles in your fingers, rather than smears, and use a small digging fork or hand cultivator to make the holes. I spent my youth fighting Phoenix's "adobe" clay in my father's back yard! :)

That's very clear/ Thankyou. Here's hoping I can find some sawdust; I know i've no coir.