Thanks for all the inputs, they pointed me into the right direction:

I've managed to work it out using this CTE Expression:

;WITH cte

AS (

SELECT distinct *

,CAST(SUBSTRING(SAMPLE_ID, 4, LEN(SAMPLE_ID)- 3) AS INT) - ROW_NUMBER() OVER (

ORDER BY SAMPLE_ID

) AS Grp

FROM (SELECT DESPATCH_ID,SAMPLE_ID

FROM [dbo].[SAMPLE_TABLE]

)A

WHERE DESPATCH_ID = 'MIR00831'

)

SELECT distinct DESPATCH_ID

,min(SAMPLE_ID) AS FirstSample

,max(SAMPLE_ID) AS LastSamp

,count(*) AS Quantity

FROM cte

GROUP BY DESPATCH_ID

,Grp