Does bachelor's degree have to be related to a PhD subject? I did a bachelor's degree in a creative industries subject (Audio Technology) because at the time, I didn't know what to do with my future so I decided to do a "hobby" (regretful, to an extent but many skills gained are transferable).

However, I am now starting my Master's degree in Computing (a conversion course that covers programming, web development, data/systems engineering, business technology and the like).

I am also gaining experience in this area also with my own projects and I already have a sub-field that I'd like to pursue for a PhD which relates more to business technology and technological consumerism - but, am open to other sub-fields also.

I have my mind strongly set to pursue a PhD after my Master's in the same field (or after a few years of industry experience), but I get a little concerned that my rather "weaker" bachelors degree subject (especially since it was split from a foundation degree to a full degree, yet I did receive a high 2:1), will hinder my PhD applications.

Any advice on how I can work around this or what I need to consider to better my applications? Or maybe I have a better chance of doing my PhD at the same university as my Master's?
I see that you question has two sides: 


The weaknesses of your bachelor topic (as you believe it is): In most of the cases, the bachelor quality is mainly evaluated for the eligibleness of joining a master program. After obtaining a master degree, there will be no concrete need to check the quality of the bachelor, rather the quality of the master thesis. 
Working in different topics/fields in bachelor, master and PhD: On the one hand, of course working on the same topic for several years makes you an expert in the field but with a very limited view. On the other hand, it is very common and normal to work on different topics. I personally know a lot of people who did their bachelors in a topic (e.g. Biology) and their PhDs in another topic (e.g. Computer Science).