Okay, I wouldn't bother telephoning the solicitors anymore. If the solicitor were so inclined, they are easier to ignore than a letter.
Write formally to your solicitor setting out those assets you just mentioned and stating that you are concerned that they yet to be account for. Refer to the letter you have received from NS&i, attach a copy and ask why you were not contacted about the bonds being remitted the solicitor and why they have held on to them for 3 months. Ask them about interest on interest-earning assets and ask them to account for that too.
Ask for detailed statements of the estate accounts so that you can check to see that all assets have been accounted for. Ask the solicitor to respond within 7 days otherwise you will lodge a formal complaint and initiate their internal complaints procedure. This will get their attention and they will certainly respond, mention 'complaint' and all solicitor jump.
In situation where you are unhappy with the services your solicitor has provided to you the Legal Complaints Service (who ultimately deal with complaints) ask that you first go through your solicitors with your complaint. If you are unhappy with their explanation then you can refer your complaint to the LCS: http://www.legalcomplaints.org.uk/home.page
If the worst become apparent (ie. you have suffered loss because of the solicitor's negligence) then you should go and see another local solicitor practicing probate and instruct him to engage with your solicitors.
Ultimately, if you are the beneficiary of the assets then you will recover the asset and I wish you the best of luck. Hopefully, it will simply be the solicitor being slack in administering the estate and not negligence.
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