Am I correct in saying there is no way on the iPhone to filter out Junk mail?
Correct. This iPhone's email client does not include a spam filter.
Also is there any way of selecting a number of mails for instance in order to delete them in one go?
In a word no but by account there is a setting to automatically remove messages that are marked for deletion and available in the account's Trash box after one day.
IMO, the bulk of any spam filtering should be at the incoming mail server for the account. For what if you access the same account with an email client on a Windows PC, Apple's Mail application and with the iPhone's email client? The junk filter on each would be different.