This decision, which will result in S4C moving out of ITV Sales, creates the only independently owned TV sales house of any scale.
It also means that ITV Sales no longer handles any third-party airtime sales contracts, except for those stations in the ITV Network that are not owned by ITV plc.
It is understood that Lench, who previously worked at TSMS, the now defunct United News & Media-owned ITV sales house, is planning to sell commercial airtime on behalf of other television stations and has already held discussions with the Hallmark Channel.
There are a number of TV sales contracts up for review. For example, the Emap television stations and Discovery are looking to move out of Sky Media.
S4C was established in 1982 by the government as the primary source of Welsh-language TV programming in the principality, transmitted on the Channel 4 frequency.
The initials in its name stand for Sianel Pedwar Cymru, which translates as Channel 4 Wales. As well as deriving revenue from advertising, it receives a large subsidy from the taxpayer.
However, S4C has suffered from declining ad revenues due to falling audiences as a consequence of increasing multichannel penetration in the region.
This has enabled viewers to watch the English-language Channel 4 rather than the Welsh-language alternatives.
Lench, who has also worked at LWT, left TSMS in 1997 to take on a consultancy role with the Welsh broadcaster.
He was not available for comment and calls to S4C were not returned.