Whenabouts did the spoken names Leicester, Gloucester and Worcester become so short? I am assuming that at one stage they were more or less say-as-you-see.

Not to mention Towcester pronounced Toaster.

An exception (are there others?) is Cirencester, which is pronounced both as spelled and as Sissester. No idea about the history of the pronunciation of any of these.

FWIW, the Oxford Dictionary of English Place-Names records the following early spellings:

Leicester: Ligera ceaster, AD 917; Ligora ceaster, AD 942, Ledecestre, AD 1086 (Domesday Book); Legrecestra, AD 1130; Leircestre, AD 1205.

Gloucester: Gleawecestre, AD804; Glowecestre, AD 1086 (Domesday Book); Glouchaestre, AD 1205.

Worcester: Ueorgorna ceastre, AD889; Wigorna ceastre, AD 779; Wigraceastre, AD 904; Wirecestre, AD 1086 (Domesday Book).

(The ODEPN is really concerned only to demonstrate the etymology of these names, not their later pronunciation; so unfortunately it only records spellings any later than these where necessary for that purpose.)

Edited to add:

Bear in mind also that just because Leicester, Gloucester and Worcester are spelt that way it does not necessarily follow that they were ever pronounced that way. Anglo-Saxon scribes consistently spelled “London” as Lunden, showing that they pronounced it just as we do; the two O’s in the post-Conquest spelling are a product of Norman scribes and Latinity.  I can quite imagine scribes who knew that the last element in those names was Latin castra insisting on giving it a value in writing that it didn’t actually have in real speech.

In the case of Gloucester, it appears that the use of Gloster - both as text and pronunciation - dates back to at least to Shakespeare’s time.  Today it is generally accepted that the spelling is Gloucester and the pronunciation is Gloster.

WS died in 1616 and no manuscripts of his plays, in his own hand, have yet been found. The first published versions of most of his works were brought out in 1623, in what is known as the “First Folio Edition.” This 1623 Edition has been electronically stored and reflects text from Richard III containing the line spoken by the Duke of Gloster.  Subsequent versions have varied in respect of spelling and other aspects.  But the First Folio is the daddy of them all!

But, the Gloster vs. Gloucester supporters are both right, because the many compositors of the Shakespeare texts over the years have each adopted spellings they believed to be correct.  Here is a brief excerpt from a relevant website (http://www.playshakespeare.com/forum/general-shakespeare-discussion/15-the-end-of-editing-shakespeare) which explains the position: “Compositor F spells the name in this way in some documents and Gloster in others.  ‘Glouster’ is Compositor B’s preferred spelling, ‘Gloster’ ‘Glo.’ & ‘Glost.’ is Compositor E’s choice.  And Compositor Q consistently uses ‘Gloster’, which reflects the proper pronunciation. (Jay Halio, Cambridge edition, 1992)”

Thousands of books have been published on the Bard’s work.  One of these is Collins’s “The Complete Works of William Shakespeare.” In an early early edition, Gloucester is spelled Gloster.  In fact, in this volume the Gloster clan is mentioned in six of WS’s plays, with nary a whisper of anyone called Gloucester.

Michael Drayton (1563-1631) was a contemporary and friend of Shakespeare.  In his “The Ballad of Agincourt,” describing Henry V’s 1415 victory over the French (verse 13) he records: 
”Gloster, that duke so good,
Next of the royal blood,
For famous England stood,
With his brave brother
Clarence, in steel.............”

Edit for typo