due diligence on flat sales within a block - no leasehold records on Land Registry

I'm thinking of buying a single unit within a large (ten-storey) block of apartments.

The estate agent says the whole block has sold out within the last 6 months, and they are down to the last few units.

I realise that not every sale may have completed, but I have been using the Land Registry search engine to perform some basic due diligence. Unfortunately I have not been able to find a single record of one flat sale within the block. The search query brings up 84 records, including the flat I'm thinking of buying, but they all refer to the (freehold) building purchase, which date back to the erection of the building 6 years ago. There are no leasehold records, hence no evidence of any of the alleged flat sales.

I'm confident in using the search engine (I have successfully found leasehold records for apartments within other developments in the UK).

Should I be surprised by this absence of leasehold records, or is it very common for there to be a several-month lag from apartment sale to appearance on the website? I thought I would find at least one sale record!

If the lag is to be expected, is there anything else I can do to verify the claims of the salespeople?

Alan - the time lag is to be expected and I suspect that the leasehold purchases are waiting to be processed at our end.
If you want to confirm that with us then you can submit a non priority search by post (form OS3) and the search result will confirm what's pending against the freehold title including each lease and parties for example