Thank you for all the suggestions (was not expecting so many so quickly!). And sorry for the delayed reply but I was away this weekend so only had chance to try some of them out yesterday.
I started by trying the tamping technique Dave suggested and it did seem better on the second shot I tried (although this shot was also fast at around 20 seconds). Perhaps I may have been grinding too coarse (as others suggested) and then overtamping to compensate so will go a bit finer with the revised technique and see how I go.
The beans were the pioneer roast from altitude coffee but I have very little left now. Originally bought 1kg about a month ago (froze half to keep fresh). I have had similar issues (although perhaps not as visually bad) with other beans.
I have a couple of coffees from Monmouth (one medium, one medium-light roast) to try and they’ve been rested about a week now so will be using those from tomorrow onwards.
Will also try to remember that it’s not all about how the shot looks! Although I was also struggling to get consistency of taste between shots which is why I suspected it was me getting something wrong.
Thanks again everyone and will let you know how I get on.