Hello Paul, I wonder if you would care to provide us with a little more detail?
I presume that by "Tower crest" you mean the tower armorial bearings ... What is the blazon of the arms borne by Sir John de Towyeir? The Plantagent arms are very familiar to those of us in the UK and I am not aware of any other family using their arms nor that they (the Plantagenets) used Tower arms differenced.
I would also be interested to learn the blazons of those other knights you mention; those of Sir Gerald Touret and Sir Geoffrey Sackville especially as I can find no similarity between the Sackville arms (basically variations on quarterly Or and Gules) and those of Plantagenet (the arms of England).
Further information on your research would be greatly appreciated.
Oh ... I hope you didn't pay the chap at askme.com a fee!