Can't answer regarding the passion flower, but personally l would hang fire on the salvia until the middle of April. I wait until there's a fairly safe chance we've seen the back of any frosts. The forecast for the next few days is generally chilly winds, and possibly frost overnight next week.
Okay - that sounds sensible. So I should cut it back then but at the right time? I wasn't sure if I should leave it as it was and growth would restart in the bare bits.
Yes, you should cut them back to just above a pair of leaves, otherwise they will flower mostly at the top and get leggier. This video from Will Dyson should help, there's a fair bit of background noise, but it gives you a fair idea
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I wait until there's a fairly safe chance we've seen the back of any frosts. The forecast for the next few days is generally chilly winds, and possibly frost overnight next week.