First off, this is Robert de Mortimer, not Roger de Mortimer. You may know of the latter, the first Earl of March, for his stint in the Tower of London and his overthrowing of King Edward II in the Despenser War, or potentially his own subsequent overthrow by Edward III, or his execution at Tyburn. Roger de Mortimer and his line are the Mortimers of Wigmore Castle in Herefordshire; the Roberts de Mortimer are also of a Castle in Herefordshire, although theirs is Richard’s Castle, and the two families do not appear to be related.
I’m afraid Robert is not quite as exciting as Roger. There are three Roberts – great-grandfather, grandfather, and grandson. Either the first or the second served in the Third Crusade. However, due to the dates, I think the third Robert de Mortimer is the one referenced here. He served several times in Wales, and might (emphasis on the might) have been involved in the death of Llywelyn ap Gruffydd, Prince of Wales at Builth Wells in 1282. He also married Joyce de la Zouche, and acquired several manors in Northamptonshire.
The Sakeville or Sackville family were local to Sussex. The Sackville manor house came into the family via the dowry of Joan Mortimer, who is either the wife or daughter-in-law of the Andrew de Sackville referenced here.
Arms of William Longspée, 3rd Earl of Salisbury 1196-1226 (1176?-1226) and Ela Fitzpatrick, Countess of Salisbury
From p95 of Lacies Nobilitie by Sir John Ferne (1586)
Blazon: Per pale baron and femme azure six lioncels or and paly of six gules and vair on a chief or a lion passant sable
Longspée was an illegitimate son of Henry II, who came into his title by marrying Ela, only child and heir to the second Earl of Salisbury, William FitzPatrick. Ferne somewhat disapproves of these arms; he is adamant in his position that illegitimate children may never bear the arms of their father. He sees even the baton sinister mark of bastardy as a grudging concession to popular consensus.
Marquis of Mortemart 1643-1663, duke of Mortemart 1663-1675 (1600-1675)
Blazon: Party of eight I gules a crescent vair (Maure), II azure three fleurs-de-lis or surmounted by a bendlet couped gules (Bourbon), III gules nine mascles 3, 3, and 3 or (Rohan), IV barry of ten argent and azure, three chevronels gules (La Rochefoucauld); V argent a serpent nowed azure, crowned or, and devouring a child gules (Milan), VI gules a chain in saltire, cross, and orle or, charged with a center point vert (Navarre), VII gules a pale vair (des Cars), VIII ermine (Bretagne); overall in the fess point an escutcheon barry nebuly of six argent and gules (Rochechouart)