I also can't find a marriage record on line for this couple. However, I think I can be of some assistance and partially answer your query. William Alexander's will can be found in the Suffolk Archives on line collection - it costs a little to access. This, at least, confirms that William's wife was called Grace, as was one of his daughters. It confirms that he must have died in 1706. It also confirms that William had a son called Samuel but there is no birth record available as of yet - you may need to visit or pay for research at the Suffolk archive?
From another set of Dikes wills from Bury St Edmunds (Suffolk Archives), it is not difficult to deduce that Martha Dikes married Samuel Alexander - there are references to Samuel being a son-in-law and a brother-in-law (the name was changed to Dykes at various stages in different branches of the family.) So far, I have been unable to find an official marriage record. Martha Alexander left a will confirming that she had two sons called Samuel and Dykes (Suffolk Archives).
The Dikes and the Alexanders were Quakers (the first Dikes to become a Quaker was Edward Dikes who died in 1664 in Bury St Edmunds). As far as can be determined, it seems that the first Alexander Quaker was the William who was married to Grace. Both Samuel and Martha's burial record can be found in Quaker records - this is also true of some of the records of their sons Alexander and Dykes and their descendants.