Thanks for all your suggestions - have talked it through with the builder and most liketly it was to do with the acros being removed too soon as suggested. Hopefully this has just lead to some settling onto the steel and their will be no further movement. The builder made good the crack before I could get a photo (suspicious?!), but at least will be able to tell if there is any more movement. Building inspector coming on Tuesday to take a look.
I did a similar job recenly, used 8 acrows for a 13' steel, left them in for 4 days after fitting the steel, no movement at all! must have been in way too much of a rush takin them out that soon
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