another way of doing it without windowed function:

SELECT pl.ID, usr.FirstName +' ' + usr.Surname as Createdby, pl.ProspectGuid, pl.MobileNumber as CTN, pl.Firstname + ' ' + pl.Surname CustomerName,

Convert(varchar(10),pl.CallBackDate, 103) as CallBackDate, pl.CallBackTime as CallBackTime

, lfd.PricePlan as Tariff

, lfd.Price

,Convert(varchar(10),lfd.ContractEndDate) as ContractEndDate

FROM ProspectLead pl

inner join dbo.UserAccount USR (nolock) on USR.ShortAbbr = pl.CallBackSetBy

cross apply (select top 1 l.PricePlan, l.Price, l.ContractEndDate from dbo.LeadFrontierData l where l.LeadID = pl.id

order by l.PricePlan, l.Price, l.ContractEndDate) lfd

where((@Responsibility = 14) or (pl.CallBackSetBy = @UserAbbr))

and CallBackDate is not null

and CallBackTime is not null

and CallBackDate = @TodaysDate

Just curious, is any particular reason why you're using NOLOCK?