Thanks everyone, plenty to think about and look into further..I think I'll try and plant some seed in plug trays and give that ago at first as it is a steep slope and I'd be worried about taking all of the grass up and the soil sliding down. I might just concentrate on a few areas rather than do the whole area as it is a fairly big section and so I'd rather do a little bit well and it sounds like I'll need to keep an eye on how well they do up against the grass!
Meadow Cranesbill (Geranium pratensis) does well even, against grass, as it is a larger plant. So does Betony (Stachys officinalis) and mine have self .seeded to give me plenty of flowers.
Most good seed merchants do a 'meadow mix', but it depends what you are looking for. Most people seem to think of the poppy/cornflower type mix, but these are cornfield flowers that germinate in cultivated soil and won't out-compete grass.
The site with the widest range is probably Pictorial Meadows but they are fairly expensive if you want something simple and somewhere like Chiltern or Plantworld should serve you well.
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Thanks again everyone