Having updated to 4.1.4 I'm now seeing a reply to this topic button on my site mobile view, is this a setting as I'm not seeing the same button on this site here? 

Yeah it's a CSS class. Since over here the tool-list has the '.ipsResponsive_hidePhone' class on it, it's hidden for mobiles devices. Check your theme/template

I'm using the default theme with just a few colour changes in custom.css, I have reverted the topic template although all I have edited it with is a custom ad location at the top but it's still there :/ 

Ooo yeah I just checked the template.. Try when logged out, or not on an admin account, at least.

The topic template tests for special permissions before it determines whether to hide the toolList on the phone or not, so a regular member shouldn't be able to see that button there..

Shows when logged in as a admin, doesn't show when logged in as a member or guest. Guess it's a bug then?

Just now, daveoh said:

Shows when logged in as a admin, doesn't show when logged in as a member or guest. Guess it's a bug then?

Doubt much concern was given to it

You CAN go to 'topic' template and change it:

{{if $topic->container()->forums_bitoptions['bw_enable_answers']}}
	<a href='#replyForm' class='ipsButton {{if $topic->locked()}}ipsButton_negative{{else}}ipsButton_important{{endif}} ipsButton_medium ipsButton_fullWidth' data-action='replyToTopic'>{lang="answer_this_question"}{{if $topic->locked()}} ({lang="locked"}){{endif}}</a>
{{else}}
	<a href='#replyForm' class='ipsButton {{if $topic->locked()}}ipsButton_negative{{else}}ipsButton_important{{endif}} ipsButton_medium ipsButton_fullWidth' data-action='replyToTopic'>{lang="reply_to_this_topic"}{{if $topic->locked()}} ({lang="locked"}){{endif}}</a>
{{endif}}

Find this little segment.. You want to edit in the 'ipsResponsive_hidePhone' CSS class onto both of the <a href...> elements, so the above bit should look like this:

{{if $topic->container()->forums_bitoptions['bw_enable_answers']}}
	<a href='#replyForm' class='ipsResponsive_hidePhone ipsButton {{if $topic->locked()}}ipsButton_negative{{else}}ipsButton_important{{endif}} ipsButton_medium ipsButton_fullWidth' data-action='replyToTopic'>{lang="answer_this_question"}{{if $topic->locked()}} ({lang="locked"}){{endif}}</a>
{{else}}
	<a href='#replyForm' class='ipsResponsive_hidePhone ipsButton {{if $topic->locked()}}ipsButton_negative{{else}}ipsButton_important{{endif}} ipsButton_medium ipsButton_fullWidth' data-action='replyToTopic'>{lang="reply_to_this_topic"}{{if $topic->locked()}} ({lang="locked"}){{endif}}</a>
{{endif}}

Pretty sure that should do it!