First you can use the 'breadcrumb' (like Hansel and Gretel) between the header and page, from anywhere in the forum.
Click on 'UK Monster Owners Club Forum', then just choose the forum you want from the list, in this case 'For Sale' to navigate to any part of the forum.
This page thread is in:
UK Monster Owners Club Forum > Web Page & Forum > Help and Site FAQs
If that doesn't help, go to 'Search' in the top Menu and select 'Advanced Search'
You can select from any of the criteria, date, forums and subforums, username, threads, posts etc.
In this example we're looking for a thread started by Stinger101, so just start typing the username and use autocomplete to select the right user
leave 'Find Threads Started by User' selected
Hit the 'Search' button at the bottom of the form and you'll then see a list of all matching threads.