I recently had a guy in to do this and he swore by taking the sawdust from the second sand (this layer was close to "proper" wood colour, unlike the first sanding), then mixing it with varnish and some pva glue. Then he filled the gaps using a filling knife (admittedly none were more than say 8mm - bigger ones than this he filled with spare "tongues" we found under the floor).
(PS my floor had originally been T+G, but years of clumsy plumbers and electricians has put paid to most of the Tongues)
Once it had all dried he did the third sand and it was pretty convincing!
Must be standard prictice , most have mine have been knackered also, looks like the spark used brute force to lift the boards, some have great big chunks missing, not just the tongues. I always thought you used a circular saw along the length of the join, that way you only ruin one board.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local