Monarch had a range of aircraft at Gatwick by 1996, and any of these could have been operated to Athens. Actually the fleet has not changed a huge amount in the intervening time.
The four A300s still in the Monarch fleet were delivered in 1990-91, and mostly operated out of Gatwick, as they still do.
There were also several A320s, which to the uninitiated can look similar to a 757 (I exclude the regulars on this forum from such a comment
). The first A321s did not arrive until the following year.
There were the 757s. Monarch were a pioneer in using these, some of which have been in service with them since 1983.
There were still a few Boeing 737-300s about, Monarch were just withdrawing them to be replaced by the A320s.