Does the time difference between London and Mumbai change during the year?

Yes. Because of daylight saving time the gap moves between −5h 30m and −4h 30m.

When is the best time to call Mumbai from London?

Between 09:00 and 12:30 in London, which is 13:30 to 17:00 in Mumbai.

London is 4h 30m behind Mumbai. This gap is not fixed: daylight saving time moves it between −5h 30m and −4h 30m during the year.

London

United Kingdom

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Mumbai

India

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A 9-to-5 day in each city overlaps for 3h 30m. That is 09:00–12:30 in London, which is 13:30–17:00 in Mumbai.

City	Time zone	UTC offset
London · United Kingdom	Europe/London	UTC+1
Mumbai · India	Asia/Kolkata	UTC+5:30

What is the time difference between London and Mumbai?: London is 4h 30m behind Mumbai. This gap is not fixed: daylight saving time moves it between −5h 30m and −4h 30m during the year.
Does the time difference between London and Mumbai change during the year?: Yes. Because of daylight saving time the gap moves between −5h 30m and −4h 30m.
When is the best time to call Mumbai from London?: Between 09:00 and 12:30 in London, which is 13:30 to 17:00 in Mumbai.