I am in NW England and back onto a farm with very few farm animals
Do you have visiting dear? The article below says these plants are generally deer resistant but they will eat anything if hungry enough.
http://sonomamg.ucanr.edu/Plant_of_the_Month/Phormium/
https://www.gardeningknowhow.com/ornamental/foliage/new-zealand-flax/new-zealand-flax-plant-care.htm
https://www.gardeningknowhow.com/plant-problems/pests/animals/camouflage-gardening.htm
I do intend to leave it for a few weeks to a month before I even do anything but eventually will add sheep or even cow manure before planting maybe a lemon tree
You can replant anytime since you didn't use an herbicide. Adding some compost or manure is a good idea to replenish the soil. Make sure you got all the rhizomes out or some regrowth may occur. Also, if it died of disease, you might consider replacing the soil.
A lemon tree needs full sun all day and you might have some shade going on in that spot.