From there the letter went to the head office of the Twopenny Post where it received
2) the date stamp framed in red, 7 0’Clock JU 21 1814 Nt. The Chief Office date stamps had the month before the day. There it also received
3) the hand stamp ‘2’ for the twopenny post charge.

It then went to the main GPO where it received 4) the double ring black circular date stamp JU 21 814 (the year in three figures in a curve) and 5) the cost of 1/1, which I think would be the 11d for the London to Middleham, (covering a distance of between 170 and 230 miles although Middleham was listed as 232 miles from London) and then the additional 2d.

There is no arrival stamp to show when it reached the delivery office.

The letter is sealed with a black sealing wax which was broken when the letter was opened, and it is hard to see anything on the seal, but it may be his initials GB.

The letter has a filing note on the outside, which is dated “June 14th 1814 Geo Brooks’s acct respecting the Wakefld Trust Business”.
This is odd as the letter inside is dated June 21st and the postmarks match that date.

So now to the letter:

Twickenham June 21st 1814
My Dear Sir,
Mr Lamb having desired a statement of the Wakefield Trust Acct I sent it to him on the 21st Ult., and he having desired me to sell the remaining 4 Exchequer Bills & pay the amount, together with the £43.18.11 the Balance of the Acct, I sent him, to Brooks Son & Dixon I have done so & sent him the Acct.

As you may like to have my Acct I send it you on the other side subsequent to the one I sent you on the 4th of Novr 1811. I hope you are got quite well & that dear Mrs Breare is quite so, and am
My Dear Sir
Yours most affectionately,
Geo. Brooks.

Pray tell Her Grace that I am exceedingly grateful for Her great Kindness to my Daughter and her Children, it is like her sweet Self.

I found out that the writer of the letter, George Brooks, was well known, and his portrait by James Heath, is in the National Gallery, NPG D32186 which cannot be reproduced because of licensing and copyright.

I was unable to find out which Duchess was “Her Grace” but two of the options could be either the Duchess of Leeds as the Duke was the Constable of Leeds, or the Duchess of Norfolk as the Duke owned Bedale.

However, a knowledgeable friend of ours in England, Dr. Bloy who has an amazingly informative website, A Web of English history
found this information about the addressee of this letter (John Breare, Esq.) in the directory and at the National Archives,

Quote
John Breare was agent for the Middleham Estate, which was bought 1670 and remained in the Wood family till it was sold 1880. He died on 25 May 1830, aged 82.

In 1824, John Breare, Esq. gave a ring of six bells and a local humourist produced this rather less than elegant verse:

Oh, Middleham is a pleasant place and seated by a moor
Where they train horses for to race that never raced before.
But in the winter time, it's often cold and dreary
So now they've got six bells to chime, put up by Mr Breary.

Dr. Bloy also told me that Bedale is near Middleham, (which was the home of Richard III in times gone by), and found an entry in History, Directory & Gazeteer, of the County of York: Volume 2 By Edward Baines about the postal system in Bedale and Middleham at the time.

The Post-Mistress in Bedale was Frances Mafham. Letters from London, York, Wetherby, Boroughbridge and Thirsk arrive daily at 50 minutes past 4 in the morning, (delivery of Letters at 8 in the morning).

A Riding Post departs at 50 minutes past 5 in the morning, taking bags for Middleham, Spennythorne, Leyburn, Askrigg, and Hawes, all Penny-Post towns, and returns at 4 in the afternoon, departs immediately with letters for Thirsk, York, London, &c.

NB A Penny-Post to Masham at 6 in the morning.

The Middleham post master was Matthew Clarkson, Office, Market-place. Letters arrive every day at half past 7 o'clock in the morning and are despatched at 1 o'clock in the afternoon, to Bedale.