Fleet numbers aren’t fixed. Here are the most common UK reasons a bus can be renumbered - even if it still looks like the same vehicle.
Some operators use different numbering ranges by garage. When a bus transfers, it may be renumbered to fit the new depot’s system.
When companies merge or rebrand, numbering systems can be consolidated to avoid duplicates.
New vehicles may be allocated the “headline” ranges, pushing older buses into a different number series.
When a bus is sold or transferred, the new operator will usually apply their own fleet number. Registration normally stays the same.
If you only know the registration, enter that and leave fleet number blank rather than guessing.