The question was whether the IMOS service had been included in the GDS contract or whether it had been maintained under the IMOS contract. The NHS could only terminate the GDS contract if the dentists were in default (which they were not). Conversely, the NHS could terminate the IMOS contract by simple notice. Therefore, the NHS argued that the GDS contract was not validly amended because the mere reference to the `provision of a . (IMOS)service” did not contain any details about the service and the entire agreement and no oral amendment provision prevented the reference to the IMOS contract to remedy this defect. The service must therefore be provided as part of the IMOS contract, which could be terminated without notice. The Court of Appeal disagreed with the NHS and agreed with the trial judge`s conclusion – but not his reasoning – that the IMOS service had been included in the GDS contract. The judge had wrongly examined contemporary correspondence, the oral statements of the contracting parties and their post-contractual conduct. In any event, the amendment corresponded to the whole agreement and not to verbal amending provisions. The original 2006 GDS contract was reportedly amended in 2009 when the parties signed a contract amendment agreement form that included an “additional service” from “reserved” to “provision of one. (IMOS) service”.

Dentists continued to offer an IMOS service until 2016, when the NHS attempted to terminate the deal. Three dentists provided general dental services to the NHS under a GDS contract concluded in 2006. It contained a full agreement clause, which was the subject of a clause without oral amendment. What it means to “provide one. (IMOS) Service”, which is now governed by the GDS Treaty, the Court of Appeal allowed extrinsic evidence, namely the IMOS Treaty, according to what is sometimes referred to as the “private dictionary principle”, in order to give meaning to this unconventional expression. Importantly, only the description of the service was included in the GDS contract and this description did not include the termination provisions. Therefore, the NHS could not end with a notice period. In NHS v. Going ahead, the Court of Appeal ruled that a contract had been validly amended and that the NHS therefore could not terminate contractual arrangements under which dentists provided intermediate minor oral surgery (IMOS) services. In 2007, the NHS and dentists each signed a contract to provide an IMOS service for 12 months. The duration of this IMOS contract expired but was continued by the behavior.

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