EDIT: I have just found the Digital UK Coverage checker website and it looks like I am lucky to be getting what I have as it seems I shouldn't even be getting Freeview???
Revisit it it and compare the first row for the 'Rowridge' section with the 'Rowridge VP' section.
If 'Rowridge VP' predicted reception for a roof aerial is a lot better than 'Rowridge' then that would explain why you appear to have a vertical aerial. (But have you checked in the loft yet?)
If 'Rowridge' and 'Rowridge VP' are both about the same then the installer might have given you a grouped aerial to give the more traditional multiplexes a boost but this would have been at the trade-off of definitely losing the other HD channels.