After coming across some facts online just a few moments ago, my interest has been piqued.
First, I discovered that although raw arsenic (alleged to have been used to poison William Longespee) was known since Greek times, elemental arsenic (more useful for poisoning) was not discovered as such until the year 1250, which is 24 years after Longespee’s death. While a rat found in his tomb contained traces of arsenic, it is difficult to control for cross-contamination since the tomb was opened in the 1790s during restorations at the cathedral; at that time, elemental arsenic was in wide use.
Hubert de Burgh may well have poisoned Longespee, but I do not believe arsenic was the poison of choice and I don’t think a natural death from illness can be ruled out since he was noted to be in poor health upon his return from Ile de Re.
Speaking of the cathedral, I discovered that the tombs were all moved into their current position during that same restoration by James “The Wrecker” Wyatt. I believe I may have mentioned that I remember screens between the columns in the church or cathedral I saw in my memories, and at first I had discounted this. I decided to contact someone at Salisbury Cathedral to see if anything is known related to screens between the columns in the nave.
By the way, I really wish there were better records of how the former cathedral at Old Sarum looked. All that remains is a ghostly outline in the grass, lost for many years and excavated (if memory serves) at the end of the 19th century. It would be interesting to know if that cathedral also had purbeck marble columns.