If you have concerns then yes it’s fair to raise them and see what they say. As mentioned by @Joy* better to do that now so it’s on record so you can agree some contingency resolution if the turf doesn’t take as hoped.
Thanks for all advice - recent rain seems to have a given a little 'lift' in that the green bits are getting greener and the affected turves have started to catch however there is still thatch evident. Contractor has agreed to come round and have a look . . .fingers crossed!
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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.