Today's update:
Went to LBS to discuss and him to look at it. He was shocked to see a failure like that. He reckons £300ish for a new carbon fork, £250 is what he has been quoted (by
Halfords) for a replacement Boardman fork. When I said that was more than I could afford he suggested getting a steel fork for £100 as he said it would be more reliable, forgiving and would last a lifetime. Unfortunately it would look completely different (the skinny tubes, small top bit).
He suggested to go to Halfords first, see what they say and then complain to Boardman on facebook. He said if it was his company he would do something about it.
We went to Halfords, they said as it is out of warranty they can't replace it but they will look up how much a new fork costs and let us know on Monday. They said a similar one was £130 but they don't know the Boardman prices.
So it looks like I might end up having to get steel purely because of price. I wanted carbon so the bike was like it was designed to be but am ok to change material if needed. But I think it would look silly if it had skinny forks on wide tube frame. So I'll have to look and see if <£300 carbon forks exist or if wide tube steel forks exist. I'll also see what price Halfords come back with on Monday.