Well, it's perfectly lawful to appoint a councillor as clerk, provided they are not receiving any payment for carrying out the work of the clerk.
It isn't unusual for a council member to fill in for a missing clerk, and often this is helpful because it provides continuity and avoids costs. Bringing in temporary staff as clerk may be difficult or ineffective. In general, very small organisations do find it difficult to cope with maternity leave, although they must honour their obligations in this respect.
Having said that, it isn't ideal for such an arrangement to persist for a long period, and it is not really desirable for the chairman to take the role of clerk. Too much is concentrated in one pair of hands for it to be a sound democratic practice. Nobody can act as clerk unless authorised by a resolution of the council, so the matter is the responsibility of the whole council.
