Sire De Grugy will face six rivals, including two Irish raiders, as he bids to complete a hat-trick of notable victories this season in the Sodexo Clarence House Chase at Ascot on Saturday.
The eight-year-old gave trainer Gary Moore a first Grade One triumph when lifting the Tingle Creek Chase at Sandown in early December and then took full advantage of Sprinter Sacre being pulled up to capture the Desert Orchid Chase at Kempton.
Somersby's sole success at the top level came in this contest in 2012, having been short-headed by Master Minded 12 months earlier. He was also third to Sprinter Sacre last winter when the race was run at Cheltenham after Ascot was lost to the weather.
The Mick Channon-trained 10-year-old will try to land an emotional victory following the recent death of Terry Biddlecombe, husband of Somersby's retired former handler Henrietta Knight.
Henry de Bromhead's Days Hotel and Shark Hanlon’s Hidden Cyclone makes the trip across the Irish Sea.
The Paul Nicholls-trained Kauto Stone, Evan Williams' Lancetto and Colin Tizzard's Oiseau De Nuit complete the seven runners.
There were three withdrawals at the 48-hour final declaration stage - de Bromhead's other entry Special Tiara, Fox Appeal and French Opera.