Teaser 2952: Silly slip

From The Sunday Times, 21st April 2019 [link]

Please help me find my silly slip. I correctly added a five-figure number to a four-figure number to give a six-figure total. Then I tried to substitute letters for digits systematically and I ended up with:

SILLY + SLIP = PLEASE

However, in these letters I have made one silly slip, so please find it and then work out what the correct sum was.

What was the correct six-figure numerical answer?

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