She had her greatest achievement at the 1977 European Athletics Indoor Championships at the age of nineteen, where after qualifying for the final as one of the fastest losers she went on to beat Totka Petrova to win the 800 m gold medal with a world indoor record-equalling time of 2:01.1 minutes. This time was a British indoor record for 26 years, becoming the longest-standing British record at one point, and was improved upon by Kelly Holmes in 2003. It remains the fastest ever time by a female British under-23 runner. That July, she ran her lifetime best for the 800 m outdoors in Oulu, Finland, clocking a time of 2:00.6 minutes, which made her the second best British performer in the event at that point, behind Rosemary Stirling. Colebrook also won the women's title at the North of England Cross Country Championships that year.