As its June 2015 Newsletter declares, HMRC aims to issue an IHT421 probate summary in ten working days from receipt of an IHT400 Inheritance Tax Account.
However this does depend on the inheritance tax being paid first. If it has not been credited to the HMRC account, your IHT400 goes into a different, and slower-moving, pile of correspondence. This can mean delay, and you may need to chase.
Where you have various financial institutions paying the tax, I’d suggest you get confirmation from all of them that all tax has been paid before submitting the IHT400.
This allows you to keep in control of the process and assists HMRC in keeping to its deadline.
Jan Wright TEP is a Director at Harrison Drury, Lancashire, and a member of STEP’s UK Practice Committee
The STEP Blog 
My most recent experience: Direct Payment made by Santander on 24th May – confirmed received by HMRC in our call to them on 6th June prior to my holiday. Still no 421 by my return on 27th June – holding for ages to speak to an advisor but being assured that would be issued within 48 hours. Still no 421 by 6th July – holding for ages to speak to an advisor who had a full record of the history and simply could not understand why not actioned yet. He explained that they are not able to intervene or remove a file from a caseworker. However he was so horrified by the delay that he suggested I fax a duplicate to him which he would receipt and send to me by first class that day. I did, He did, and it has duly arrived!