Executing stored proc from DotNet takes very long but in SSMS it is immediate

I have a stored proc on SQL Server 2000 that takes 3 parameters. When I call the stored proc from DotNet using SqlCommand.ExecuteReader () it takes about 28 seconds.

When I run the same query inside SSMS directly it returns immediately.

When I take the query out of the stored proc and run it directly using DotNet it also returns immediately.

These are the results from a SQL Profiler session

SP Inside Dot Net

• Duration: 28030
• Reads: 2663365
• Writes: 0

SP Inside SSMS

• Duration: 450
• Reads: 23535
• Writes: 65

Query directly inside Dot Net

• Duration: 360
• Reads: 24865
• Writes: 57

The following things stand out to me:

• The stats for the SSMS and direct query in Dot Net are very similar
• The Dot Net SP one does a huge amount of reads but no writes
• The other two make very few reads, but a couple of writes

Any help would be appreciated.

Here is a slightly obviscated version of the SP:

I doubt that it is a query plan issue because even if I run it repeatedly from DotNet, I always get the same results.

Here is a version of the SP that's been altered slightly because of IP issues. I hope it still makes sense:

SELECT 
t1.pkiOrderID,
t1.fkiBasketId,
t1.sOriginBasketCode,
t1.dtDateCreated,
t1.sOrderCode,
t1.fkiUserCde,
t1.fkiOrgCde,
t1.sApprovalPerson,
t1.dtDateApproved,
t1.sRequestNo,
t1.dtRequiredDate,
t1.Requestor,
t1.OnBehalfOf,
t1.OrderDesc,
t1.OrderTypeId,
t1.fkiAgentID,
t1.fkiAgentRegionID,
stat.iStatus,
count(oi.pkiOrderItemId) as OrderItems,
count(wf.fkiOrderId) as WorkflowCount,
t1.Currency_Id,
t1.ExchangeRate,
t1.ref_odr_idn,
t2.sOrderCode as ref_odr_cde,
t1.ref_rfq_nbr,
t1.ref_rfs_nbr,
t1.ref_doc_nbr,
t1.ref_rsn,
t1.ref_forip_cde,
t1.ref_fa_nbr,
t1.odr_sub_typ
FROM