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Just popped out to have a look at mine, growing close together in the garden. You're of course right about the number of sepal lobes, but they are broad in Philadelphus and very narrow in Deutzia. The leaves are similar in shape, the Philadelphus is a bit wider. The Deutzia leaf has numerous teeth ending in a sort of blob - hydathodes? and is only hairy along the veins on the back, while the Philadelphus has a hairy edge with only a few hydathodes and is hairy all over the back. The venation pattern is different too, with the Philadelphus more open and branched. Thanks for getting me to take a closer look.
As there is no mention of Deutzia in Stace I thought the following may be of some use in separating Deutzia from Philadelphus.
The online Flora of China distinguishes the two genera in its key as follows;
Deutzia - Leaves often stellate hairy; petals 5; stamens 10(-15); capsule 3-5-valved, loculicidally dehiscent
Philadelphus - Leaves not stellate hairy; petals 4; stamens 20-40; capsule 4-valved, loculicidally or septicidally dehiscent
... is in the 3rd edn of Stace. His key is
Philadelphus: petals and sepals 4; styles 4, united > 1/2 way; stamens >= 20.
Deutzia: petals and sepals 5; styles 3, more or less free; stamens 10.
Apologies to everyone for that, I was using a digital version and found no returns when I typed in what I thought was Deutzia, but most likely misspelled it. This should teach me to use the index and not take short cuts.
Lat/Lng: 53.3, -1.6
OS grid ref: SK3188