Headrest
I'm currently looking at Mazda MX5 seats for my 1200 convertible - they are supportive with integral headrests (some with stereo speakers built in!!) cheap to buy and only need slight modification to the runners to fit (there's a kit to do this for around £35) They come in a range of colours and suit the car very well, looking modern but not out of place. Quoted prices are £60 - £120 from local scrapyards with leather ones around £150. Plain black vinyl with cloth inserts will suit a Herald very nicely.
Picture of a 13/60 with headrest based on US specs:
Pictures