Dig up and reseed. But there are many different mixtures of grass seed so it might not match when you've finished.(Unless you seeded the lawn yourself and can remember which grass mixture you used)
Its not easy from the photo to be sure how big the patch is but another solution if you can do it is to steal a bit of turf by reducing the size of the lawn, perhaps by rounding off a corner or something, and use the turf you remove to patch where you took the invader from.
There is nothing I know of that you can do to prevent stray seeds taking root in your lawn as you don't mention the presence of a bird feeder - if that was the source you could move it away from the lawn. You can't really stop birds landing on your lawn, and who would want to?

Thanks, Steragram.
There's about 20 patches on the lawn, all of a similar size, about 15- 20 sqins. So it's just a matter of digging up and re-seeding? I've lived in same house for nearly 20 years and it is very strange that the 'invader' has only just appeared. No, we don't have a bird feeder and as you said who would want to stop birds landing on your lawn?!

In that case forget what I suggested! You could try contacting the company Green Thumb who have several franchises in Wiltshire - you can find the details on the web. They are lawn care specialists and will come to you either when requested or on a regular basis. They would identify the problem and either treat it or tell you what to do - not free but effective. They cared for a large lawn for us at our last house and did a great job.

I had Green Thumb looking after my lawn a few years ago. Did a good job; think I will contact them again.
Thanks, Steragram