Why are some clients being assigned wrong DP?

Overview

SCCM 2012 SP1, One Site, One MP, ~80 DP's
GLCM001  - Site Server, MP, DP in data centre
UKASHI001 - DP in branch office
UKGLAS001 - DP in branch office
One to one mapping between AD Site (or IP Range), Boundary group and DP. No Fallback DP's. No distributing content to preferred DP's.

I've been experiencing some issues with content being pulled by the end client from GLCM001 instead of the DP within the defined Boundary.

We run our domain from two data centres with ~80 branch offices.  In one of the data centres I have my SCCM Server (GLCM001  - Site Server, MP, DP etc), in each of the branch offices I have a DP.
I'm aware that boundary overlapping is the most common cause for this issue, I looked at the defined boundaries but couldn't see an overlap, and I haven't come across any log to prove otherwise. Normally I just use AD sites to map boundaries but for troubleshooting purposes I changed the boundaries of the sites I was using for testing to be IP address ranges, which made no difference.

My problem is appearing in some branch offices, but not all, and all clients within a branch office react the same.

I ran two tests
The first was to check that if the content was only on GLCM001, clients not within it's boundary didn't download content from it.  This worked as expected, the two test clients didn't download content.

On the second test, when the content is not on GLCM001, but on the DP's near the client, UKGLAS065 gets it content from the GLCM001 DP

Any ideas why?

Test 2 - Content only on DP's UKASHI001 and UKGLAS001

Client:
UKASHI013

Client LocationServices.log: 
Current AD site of machine is AshingtonLocationServices04/02/2014 11:32:51884 (0x0374)
Calling back with the following distribution pointsLocationServices04/02/2014 11:32:51884 (0x0374)
Distribution Point='http://UKASHI001.domain.local/SMS_DP_SMSPKG$/PR100087', Locality='LOCAL', DPType='SERVER', Version='7804', Capabilities='<Capabilities SchemaVersion="1.0"><Property Name="SSLState" Value="0"/></Capabilities>', Signature='http://UKASHI001.domain.local/SMS_DP_SMSSIG$/PR100087', ForestTrust='TRUE',LocationServices04/02/2014 11:32:51884 (0x0374)
Calling back with locations for location request {99F6E76F-4D7D-4946-9A88-33B7503789BD}LocationServices04/02/2014 11:32:52884 (0x0374)

Server MP_location.log:
MP_GetContentDPInfoProtected (PR100087,1,PR1,<ServerNameList><ServerName>UKASHI001.domain.local</ServerName></ServerNameList>,SMSPackage,00000000,domain.local,domain.local,<ClientLocationInfo LocationType="SMSPackage" DistributeOnDemand="0" UseAzure="0" AllowWUMU="0" UseProtected="1" AllowCaching="0" BranchDPFlags="0" UseInternetDP="0" AllowHTTP="1" AllowSMB="1" AllowMulticast="1"><ADSite Name="ASHINGTON"/><Forest Name="domain.local"/><Domain Name="domain.local"/><IPAddresses><IPAddress SubnetAddress="192.168.240.0" Address="192.168.240.69"/></IPAddresses></ClientLocationInfo>)

Client:
UKGLAS065

Client LocationServices.log: 
Current AD site of machine is GlasgowLocationServices04/02/2014 11:51:544588 (0x11EC)
Calling back with the following distribution pointsLocationServices04/02/2014 11:51:544588 (0x11EC)
Distribution Point='http://GLCM001.domain.local/SMS_DP_SMSPKG$/PR100087', Locality='LOCAL', DPType='SERVER', Version='7804', Capabilities='<Capabilities SchemaVersion="1.0"><Property Name="SSLState" Value="0"/></Capabilities>', Signature='http://GLCM001.domain.local/SMS_DP_SMSSIG$/PR100087', ForestTrust='TRUE',LocationServices04/02/2014 11:51:544588 (0x11EC)
Calling back with locations for location request {04144A8E-D780-42DE-8FEE-80EFACA4943F}LocationServices04/02/2014 11:51:544588 (0x11EC)

Server MP_location.log:
MP_GetContentDPInfoProtected (PR100087,1,PR1,<ServerNameList><ServerName>UKGLAS001.domain.local</ServerName></ServerNameList>,SMSPackage,00000000,domain.local,domain.local,<ClientLocationInfo LocationType="SMSPackage" DistributeOnDemand="0" UseAzure="0" AllowWUMU="0" UseProtected="1" AllowCaching="0" BranchDPFlags="0" UseInternetDP="0" AllowHTTP="1" AllowSMB="1" AllowMulticast="1"><ADSite Name="GLASGOW"/><Forest Name="domain.local"/><Domain Name="domain.local"/><IPAddresses><IPAddress SubnetAddress="192.168.146.0" Address="192.168.146.81"/></IPAddresses></ClientLocationInfo>)