A Perth Cup start for Martinvast hinges on how highly West Australian handicappers rate his second in the Christmas Cup.

The former French galloper achieved his most significant result in 15 Australian runs with a fighting second to Permit in Saturday's Listed race at Randwick over 2400 metres, the same distance as the Group Two Perth Cup on New Year's Day.

Trainer Gerald Ryan said the Perth feature had been a long-range target for the five-year-old but the Tattersall's Club Cup at Randwick on the same day was an alternative.

"I won't be able to find out what his rating is until Monday," Ryan said.

"There's no point going all that way unless he is certain to get in."

Even if he does get in, the forecast for hot weather in Western Australia has also prompted a few second thoughts.

"We were all for it but we're a bit hesitant now because of the hot weather," Ryan said.

"The reason we stopped with him in the winter was we worked out he was better on firm ground and we know he will get that in Perth."

Martinvast was slow to jump in Saturday's Christmas Cup but when there was no early speed, apprentice Damien Thornton opted to take him to the front.

The move almost came off with the five-year-old kicking resolutely to finish in front of all but Permit, who broke a 20-month drought from the winner's stall to score.

If Martinvast remains in Sydney he will face a rematch with his Christmas Cup conqueror in the New Year's Day feature.

Thornton, who rode 25 metropolitan winners in Melbourne last season, will join the Sydney ranks full-time after a three-month stint with Ryan at Rosehill.

"All the paperwork is done and Damien's indentures will be transferred to me from the 23rd of December," Ryan said.

"He is a really good kid and I have known his family for a long time.

"He does his homework and he has really settled in to Sydney well and I'm sure he can go a long way."