Tokyo - England's Ben Spencer appeared almost apologetic on Thursday not to have a "gone fishing" story up his sleeve after flying to Japan as cover for this weekend's Rugby World Cup final.

The Saracens scrumhalf had been taking care of his children, having watched last week's stunning 19-7 win over the All Blacks on television, when he got the call to pack his bags.

"I've not really had the chance to process the whole situation yet," said Spencer, who arrived on Monday as back-up to first-choice number nine Ben Youngs after Willi Heinz suffered a torn hamstring in the semi-final.

"I was just at home, feeding the kids, then got the phone call," he added before Saturday's heavyweight clash with South Africa in Yokohama.

"They are a bit too young to understand, but my partner was thrilled. They actually got here this morning, it's an exciting week for all of us."

Spencer was named on the bench on Thursday for the World Cup final, replacing Heinz, the only change to the match-day 23 that dethroned defending champions New Zealand in the semis.

"When I got the call it was a bit of 'right, let's get ready for a World Cup final!' It's been a crazy few days, a brilliant few days," he said.

Spencer's late call-up has been compared to that of former All Black Stephen Donald, who was flown in at short notice to join New Zealand's 2011 World Cup squad and kicked the decisive penalty in the final against France.

Then New Zealand's fourth-choice flyhalf, Donald was famously fishing when he got the call to report for All Blacks duty, and Spencer lamented that he could not come up with a similarly colourful tale.

"I don't quite have a Stephen Donald story - you know, on a boat, fishing," smiled Spencer.

"It's been mental really - but I've loved getting back in with the squad."

After being an unused replacement in New Zealand's 2011 semi-final victory over Australia, Donald replaced the injured Aaron Cruden in the 34th minute of the final, trotting on in a jersey that appeared several sizes too small to become a national hero.

England coach Eddie Jones, who has taken the 2003 champions to the brink of a second world title, four years after they suffered the humiliation of exiting at the pool stage as tournament hosts, promised that Spencer would have no such wardrobe issues.

"He'll have a shirt that fits him, so that's one significant difference (with Donald) - and he hasn't been whitebait fishing, so that's another difference," said the Australian.

"It's a great opportunity for the kid, he's excited about it."

Spencer, who helped Saracens capture an English Premiership and European Cup double this year, faces a baptism of fire against the Springboks.

"It's every kid's dream to play in a World Cup final and for me to be out here hopefully on Saturday, it's amazing," he said.

"The good thing was that I'd spent quite a lot of time with the squad in pre-season, so I'm not too out of the loop," added the 27-year-old.

"It's not been too difficult to get up to speed - Benny and Willi have helped me out a lot. It's gutting for Willi because he has worked so hard."