Hi Rhodie,
Further information found at Kingswood Archives in Sydney (State Records of NSW) from the Colonial Secretary's correspondence.

It would appear that it took months to round up all of the 6 martyrs after the pardon. They all eventually arrived at Hyde Park Barracks but somehow after that when the boat was due to sail from Sydney James Hammet got himself arrested at Windsor (50 miles inland) and missed the boat. Even if he only got a short sentence for his assault charge it have been just enough to miss the boat Given the others arrived back in England in March 1838 and his letter is dated 26 Aug 1837 he can't have missed it by much.
Given he was reportedly sent back from the Canberra (south west of Sydney) area by E J Eyre to Hyde Park Barracks (Sydney), I don't know what he was doing in Windsor which is due west of Sydney. He would not have needed to pass through Windsor on his way back from Canberra.
12 June 1835
From Superintendant of Convicts
All the Dorchester lot ordered back to Hyde Park Barracks
14 Jan 1836-re all the Dorchester convicts
With ref to your letter of names and dated as per margin, respecting the undermentioned convicts, I now beg to annex the particulars of their cases and acquaint you, that their assignees have been written to concerning their conduct, each of whom have reported favourly of them with the exception of the matter of James Hammett (Mr E J Eyre) from whom there is no reply, which has occasioned the delay in not replying to your several communications referred to but against whom there is nothing predredicial (prejudicial??) on record in this office
3 Feb 1836
re order to return to Hyde Park Barracks -Stanfield brothers at Port Macquarie
1st May 1836 from Edward John Eyre
I have the honour to request that a man may be assigned to me in the place of the prisoner named in the margin (James Hammett) and who was sent in to the Hyde Park Barracks at the order of the Colonial Secretary
6th May 1836- re James Hammett
That the latter named convict has arrived at Hyde Park Barracks from Woodlaws in Argyle the residence of his assignee Mr Edward John Eyre
26 Aug 1837
To his Excellency Sir George Gipps Governor of NSW and dependencies
The Humble memorial of James Hammett there with
That your memorialist is one of the Dorchester labourers tried in 1834 and arrived in the colony by the ship "Surrey" under sentence of Transportation .
That memorialist received his pardon in 1836 with other labourers sent there under the same circumstances who obtained a free passage to England .That your memorialist would have obtained the same indulgence but he was unfortunely detained in Windsor charged with an assault when the labourers left the colony.
That your memorialist is now anxious to return to his native land and he trusts your excellency will take his case into humane consideration and order memorialist a passage to England which he unfortunely lost on the former occasion and your memorialist is in duty bound.
Will ever pray
James Hammett
27 Feb 1839
In compliance with the direction of his Excellency the Governor contained in your letter of the 23 rd instant the prisoner named in the margin has been provided with a passage for England in the ship "Ewuretta" under an agreement which I had already made for the conveyance of Invalids, She is reputed to sail the 8th proxima .
I have the honour to be your most obed servant,
? Miller
The question now is what did James Hammett do during the 2 years it took for him to eventually sail back to England. If he was detained in Windsor jail I think we are out of luck as I have been told all Windsor jail records have been lost. Is the spelling of the ship's name (Ewuretta) correct?
I will continue investigating and see if I can find out what he did in the 2 years it took to get him on another ship. Unfortunately a lot of Australian convict records were destroyed many years ago in Sydney but we may be lucky. It might just be that the length of time it took for letters to get backwards and forwards from Oz to England has caused the delay.
Andcarred