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Many thanks for all interest and comments - what you say really confirms what I thought. I did wonder if parish records, usually handwritten, may have something more added, just to confirm that they were twins. Their dob is identical on other documents but I can't find them together, as it were, on birth records.
Ann - thanks for your offer - my local library computer service have a subscription to Ancestry and I'm able to take advantage of that so I'll keep plugging away and hope something turns up.
Thanks again.