William Fowler, born about 1400, was buried in St Dunstan's chapel at Westminster Abbey according to information contained in the will of his son, Sir Richard Fowler (c.1425-1477). The will says "I will that there be a monk or priest found to syng daily in the chappel of St Dunstan in the Abbey of Westminster where my fader [father] lyeth buried..." (PCC wills 32 Wattys).
This chapel was off the cloisters, not in the Abbey itself. The site was to the east of the area of the Undercroft (formerly housing the Museum) but did not survive as a chapel after the dissolution of the monastery in 1540. No record of this burial has so far been found in the Abbey archives nor is there any evidence of a grave in the area of the old chapel.
According to a history of the Duchy of Lancaster William came from Buckinghamshire and was a Justice of the Peace 1441-66 and perhaps the Member of Parliament for Wycombe 1431 and was father of Sir Richard Fowler, Chancellor of the Exchequer. For the good services of his father Thomas to King Edward IV William was given Preston manor in Buckinghamshire in 1465. Richard acquired this manor in 1467. William's wife was Cecily Englefield, a co-heiress of Nicholas Englefield (d.1415) who was controller of the household of Richard II. So we assume William may have died between being given the manor in 1465 and at least before 1477 when Richard died. He may have died by 1467 when Richard inherited the manor. Some histories say William died in 1452.
Sir Richard Fowler, of Hambleton in Buckinghamshire, held many other posts including Under-Treasurer of England, Chancellor of the Duchy of Lancaster and king's solicitor (the first to hold this post). He died on 3rd November 1477 and was buried at St Rumwold's church in Buckingham. His widow was Joan (daughter of Henry Danvers, mercer of London). Richard's brother Thomas was mentioned in his will.
The will is at the National Archives and a copy can be ordered via their site. Please note Richard's surname was Fouler in his will so this spelling should be used for the search.
"History of the Duchy of Lancaster" vol. 1, by Robert Somerville (1953)
Victoria County History of Buckinghamshire vol.3, p.484-5 and vol.4 for Preston Bissett manor