Hi Nick,
The thankyou.php page is specified in this bit of code (just before the DOCTYPE declaration) usually at or near line 34:
//Send Mail All Entries
if ("index.php"!="") {
header("Location: thankyou.php");
}
If you haven't changed that, then it may be that another problem is preventing the redirect.
Could you zip the contactus.php page and upload it here? It might help also if you could provide a link to the page.