Desperate measures:
"AW Ship Management Limited (AWSML) has identified and contacted the vessel ‘Ocean Observer’ owned by British company Gardline to transport approximately 20 RMS passengers from Cape Town to St Helena."
'OCEAN OBSERVER' TO BRING PASSENGERS TO ST HELENA « St Helena
Update: only 10 passengers on the Ocean Observer.
http://www.sainthelena.gov.sh/shipping-update-2/
Meanwhile, Atlantic Star appear ready to offer an Avro RJ100 interim solution:
"The team at Atlantic Star are aware of the ongoing difficulties with the Royal Mail Ship and also the limited availability of the runway at Ascension. We are working with our industry partners to form a potential solution using the Avro RJ aircraft that we demonstrated successfully on St.Helena in October 2016."
http://www.atlanticstarairlines.com