Should the NHS mix Pfizer and AstraZeneca Covid jabs?

Should the NHS be mixing vaccines for better effect, or at least offering people who have had one dose of the AstraZeneca vaccine the choice of having the Pfizer vaccine for their second shot?  The question arises because that is exactly the regime which has been followed in Germany since the risk of clotting from the AstraZeneca vaccine became evident. When the German authorities made the decision to administer the Pfizer vaccine in place of a second shot of AstraZeneca there was not much evidence as to whether this would be an effective strategy.  But a study from a group of hospitals in Berlin suggests that a shot of AstraZeneca