Thanks for your reply.
I agree, there are lots of ways to do what I want, but this would have been the easiest.
To have a file called “splash.html” means I have to code it to go to splash.html first and then also set a cookie or of some sort to say whether they have seen the splash page (so they don’t see it again).
Its quite a bit of work considering I could have just used an index.html file and then for all wordpress homepage links, go to index.php as it should do.