Alastair Cook admitted England had been left with "a lot of unanswered questions" at the end of their Test tour of South Africa.

It was not so much England losing the final Test that worried Cook. They had already wrapped up the series and were, he thought, a little lacking in intensity as a result.

The worry, for Cook, was that even when England won, they were overly reliant upon a few key players and that most of the holes that existed in the line-up before the series, remained by its end.

While the middle-order of Joe Root, Ben Stokes and Jonny Bairstow all scored heavily, the others members of the top seven struggled. Alex Hales, with an average of 17.00, James Taylor, with an average of 26.57, and Nick Compton, with an average of 30.62, all failed to take the opportunity to cement their places and may, as a consequence, require a strong start to the county season to retain their places in the side. Cook, too, scored just one half-century and averaged a modest 23.

For a team that has aspirations to reach No. 1 in the Test rankings, it was reminder that they have a long way to go. And it left Cook accepting that none of Hales, Compton or Taylor had "totally convinced" him.

"There are still a lot of unanswered questions in our top seven batting," Cook said. "There are certainly places up for grabs.

"The output we've had in this series hasn't been good enough if we're trying to get to No1 in the world, which is the ultimate aim.

"As a batting unit we've relied too heavily on Joe Root, Ben Stokes and Jonny Bairstow. Absolutely we've got room for improvement."

"At the end of the day, results matter and your end column of runs are absolutely vital. So to say they've totally convinced me would be wrong."

Cook also suggested that Bairstow had "a serious amount of work" to do to improve his keeping after a series in which he was unable to take seven chances. But, while accepting that England "can't afford" him to make as many mistakes, Cook did off-set his criticism by reflecting that he had "come of age" as a Test batsman and compared his keeping to that of Matt Prior, who improved hugely after an uncertain start to become a highly reliable performer for England.

"Jonny has been outstanding with the bat in this series," Cook said. "He's kind of come of age and proved he can bat at this level.

"He knows he has a huge amount of work to do on his keeping. We can't afford to keep putting down those chances that he put down in this series.

"It's no coincidence the two games we've won we've caught everything and the draw and the loss we dropped far too many chances. That is a big area of concern for the side but there's no reason we can't put it right - it just takes a hell of a lot of hard work.

"But I remember a certain Matt Prior having a tough series in Sri Lanka. All he did was work and work and work and amazingly no-one ever talked about his keeping.

"He was an outstanding keeper, but for the first 20 Test matches people doubted him. He put in a serious amount of work and Jonny has that ahead of him. He is the first to know that."

Prior was actually dropped for about 12 months after that series in Sri Lanka. But, after returning to Sussex and working hard, returned to the side with a much stronger game. It may be that several of this England side face a similar period back in the domestic game.

"Those guys have got to continue working hard and make sure that, when selection comes round for Sri Lanka, they have scored a bucket load of runs for their counties," Cook said.

The other concern for Cook was his side's habit of losing the final match in a series. It has now happened in seven of the last eight series and, while some of those matches have occurred in dead rubbers, it has certainly not always been the case. Indeed, in 2015, England lost final Tests to West Indies, New Zealand and Pakistan when the series was very much still live.

"It's not the way we wanted to end the tour," Cook said. "This whole five days we haven't quite been as good as we have been in the other three games.

"I can't quite put my finger on why. We spoke about how we wanted to unbeaten. We just haven't been good enough in this game.

"This week has shown us that, if you're not absolutely 100% on it, mentally in particular, then you just can't compete at the level you need to.

"We've produced a very limp batting performance. There's no excuse for that. It's disappointing for the English supporters to see seven wickets in just over an hour but overall I can't fault the way the lads have gone about this tour. They've played some brilliant cricket.

"A lot of time, under pressure we produce outstanding performances. This is just a blip and incredibly frustrating.

"The dead rubber issue is one I don't mind having if we've just won the series, but it just shows we've just still not managed to put a whole series together. I thought we would have made a really big step forward if we had played four good Tests here. We couldn't quite manage to do it.

"It just shows how much work we've still got to do as a side. We never pretended we were the finished article. We've got areas we've got to get better at. You have to give the guys a little bit of leeway. But, for us to be at number one, we do need to start gutsing some of these games out."

George Dobell is a senior correspondent at ESPNcricinfo