I think from the prospect of retention:
a clear career path needs to be identified with the staff with milestones/goals along the way - showing them that they won't have to move to get that upgrade.
Granted, this all has to be agreed with your Directorate Manager/Accountant but should you lose a band 6 for example, if you feel unable to recruit at that level then get the banding ring fenced and say employ a trainee at band 3/4 with whatever experience you feel is right and show them by achieving certain milestones they can achieve band 6 in X years with the directorate making a cost saving every year until the ring fenced band is fully achieved.
With the revamped Training Scheme now being launched I think it is an ideal tool to help people recruit new starters and develop them into fully fledged techs.
The tricky bit is working with your manager/accountant so they don't try and do the dirty on you and mysteriously remove the chance of progression along the way - get everything in writing (speaking from experience)
Ian Wilde