John - carefully remove the old stuff and use it as a cutting pattern on the new (offcuts). Usually, the bit covering the tread will have been fitted first then the band (or strip) covering the riser fitted last. To remove the old stuff first take off the riser section.
Easy enough to do.
Oh yeh, and when the new bit is fitted control the dog, or at least give it it's dinner on time
John - you wrote " ... the offender is no longer with us. My wife actually likes it being there as it provides a personal memorial to him". Well, why not give your Wife a couple of suprises this weekend - fix the carpet and ... get a copy of the photo you posted framed then hang it on the wall above the repaired section? Every time you climb the stairs you'll get a lovely reminder of your loyal, but distructive friend.
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