The harder the soil, the harder it will be for the grass to thrive in it. Where do you live? Maybe a grass that spreads aggressively like Bermuda would be helpful, you can fix the holes and it can spread in.

I live in the south of the UK.

I think the lawn is past filling the holes, it's about 50% holes at the moment!

I'd like the dig up what's left of the current grass, and then either flatten the soil that is there, or replace with a tougher top layer... then sow a grass seed that will be super hard wearing.

Problem is I'm not sure how compact I can go with the top soil, and what seed to use.

Great thanks reeljake..... I will have a look into finding a sharp sand supplier, sand sounds like it would be less firm than soil though?

And good idea on soccer pitch, I'm sure I can find some of that locally. I'll have a look at what conditions Bermuda grass needs, but I suspect the cold and wet UK isn't good enough!

Beach sand is soft, but that's not what you want. I'm talking about a sharper firmer sand like what a stone Mason would use. You should be able to squeeze it in your hand and make like a snowball--It won't be very tight, but if the sand just flows out of your hand without any structure, the grains are too rounded.

Ah ok, makes sense. Thanks :)