Im considering taking a 20 hour TEFL course which takes place over just two days. Will the certificate/reference from the course be of use when applying for teaching jobs abroad?

yes, without experience...the certificate will be useful. However, not all schools who hire ESL teachers require that they be certified. They only ask you have a BA degree in any field and that you are a native speaker of English. South Korea is looking for native speakers who will come and teach there right now. They are hurting for teachers and you don't need to be certified. So you might want to check that out, but a good website that has more information as far as what is available in other countries as well as South Korea is www.eslcafe.com check out the job board. There is a lot there. Good Luck.