A bit of a Bodge, but it will do the job;
Use this link http://www.mx5oc.co.uk/forum/r.ashx?3 and it will automatically log you in, and return you to the main forums screen, or present you with the login screen if there is genuinely no trace of who you are (if you don't click the 'remember me' box, or delete your cookies for example).
OR, you can of course use http://www.mx5oc.co.uk/forum/forums/TopicsNotRead.aspx to go straight to the not read page - this should also log you in automatically
If you currently use the links toolbar, or favorites to access the forum, just replace the current address with the one above. It's not an ideal solution, but I'm sure everybody will admit it is better than nothing...
If you don't use any sort of link, and type the address each time then this is not an ideal solution, but hopefully the developers can get something proper quite soon - failing that, make more of a commitment and get the site in your favorites!
If you are accessing the forum through the main OC site, then you should not have an issue. If you do, login, and logout, close your browser, and login to the OC site here, and check the box - it should work fine that way. Only the main forum page(s) is/are not checking the cookies.