Well the main thing would be the
SEO
PHP Code:
clio16valver.co.uk/forum/login.php?redirect=viewforum.php&f=1&start=0
Links like this tell google nothing so you would need a decent rewrite script the best is VBSEO.
Also with VB's new Yahoo YUI posting scripts the speed of the forum is a lot faster. Most phpbb forums reload the page when you post which can be very annoying after you've got used to Ajax posting on a VB board.
Generally they charge because it's the best, do a
google search for a forum subject and you can guarantee the top results will be VB. A forum's only useful if people are finding it like most sites.
You have a nice base in that you've got lots of posts and members but judging by your Alexa rank the traffic through the site is very low not even in the top million I would bet this would increase in a matter of months with VB and VBSEO installed.
Also this is a major problem.....
link:http://www.clio16valver.co.uk/ - Google Search
Google hasn't registered many reputable back links to the site and 0 to the forum which means it's link power will be very week.
I also don't know of any Sitemap plugin for PHPBB (I could be wrong) but I have a sitemap plugin for Google and Yahoo which means when a new thread is made the forum tells google about it straight away meaning the thread can be in google's index in under 3 hours which is really important.
My advice would be if you've got the money to invest, start from scratch and build from there. Buy VB 3.7.0 and VBSEO buy a new domain which states exactly what the forum is about. Although the name you have is pretty funky a boring domain like clio-forum.com would be ideal for upping your traffic.
I would say the new image upload and social features that are being built into VB would be ideal for a car forum, you can open social groups and upload images of your cards etc. Although these features are very new they will be expanded on in future releases.
There's new ad placement format in 3.7.0 as well which will be great to earn you a bit of revenue and pay for your hosting and the cost of VB. Also the spam and user banning options on VB are top of the range meaning when you get successful and the spammers start turning up it's easier to ban them for good.
You can also get a blog plugin with VB that integrates users blogs into the forums meaning each member can keep their own blog so you will attract traffic not just for the car posts on the forum but also the posts your members are making on their blogs.
vBulletin® - vBulletin Blog Features
It does seem like a big payout at first but when you've got alot of traffic and you're making a bit of money from advertising you'll never look back, also the support is great from VB.
Hope this helps mate, if you need anything drop me a PM. Best of luck.