You will have to ring round a few local law firms (concentrate on one who specialise in family law) and ask. Charges can vary. Google will be the best place to start.
You may struggle to find a firm to do it for free (even though the letter may not take long to write there is a considerable amount of administration involved in opening up a file, dealing with ID formalities and other compliance checks) but you could ask if it could be done on a fixed fee. Then you will know the financial damage before proceeding.
I hope this helps.
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