Schmidt Technology easyFLOW 9000M refill to ISO 12757-1 G2 Made in Germany. A substitute for Parker refills.
After considering parker (roller) ball refills as too expensive and the substitute ones at £1.50 in shops and online venders of Indian and Chinese
sourced ones with bad reviews re. small capacity, intermittent writing I found this source which with postage for a pair is still cheaper than retail shops. Writing is more fluid but slightly wider than the medium of a Parker from UK which I assume is geniuine as it came with the original pen. Conclusion highly recommended.