Sink Rate,
In the UK, level restrictions on STARs are placed there for flight planning purposes so that crews may plan their descent profiles and fuel loads. They are
not cleared levels and there is a plain language message on the STAR charts which states that actual levels will be assigned by ATC.
Now if ATC have cleared you down to FL180 level by ABCDE (which matches the published profile on the STAR) and then subsequently reclear you to FL140 by VWXYZ but do not mention the original restriction, then that original restriction is automatically cancelled.
SIDs are subtley different in that the restrictions published in them are actually part of the ATC clearance (a SID is in fact just an ATC clearance which is charted to cut down on RT). These restrictions are placed there to ensure terrain clearance, noise preferential routeings, and ATC procedures are all complied with.
Strictly speaking, ATC clearing you above the SID final altitude would by default cancel all other restrictions in the SID, however as the pilot you are still responsible for ensuring that you follow the noise preferential route (and altitude elements) plus maintain terrain clearance. As most pilots are busy after departure I would imagine they would just continue with the SID restrictions rather than work out for themselves the levels they need to be at to ensure compliance.
Examples of SID & STAR charts from the AIP can be found here. Just pick the airfield you want. The references to levels, NPRs, and climb gradients for terrain purposes are contained within the notes of each chart.
http://www.ais.org.uk/uk_aip/pdf/ad/aipad2.pdf
'Unrestricted climb/descent' is not in UK pharaseology.
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PPRuNe Radar
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