Can’t give specifics on DHL although have heard they are more thorough than some on who they take on, but then that might depend on the site.
Suspect there won’t be any written tests and it’ll be more about making sure you can drive a class 2, checking your licence is valid (take your national insurance number to all agency jobs), and if they need people then showing you round the depot and saying here’s the keys. Just make sure they know you’re a new pass so they might give you pointers a least as to anything you miss.
One thing I would mention is sometimes agencies will send lots of people to a job for assessments just so later they’ve got plenty of options when work comes in at 4am and they need to ring someone. A lot depends if your agency are the main contractor for that DHL site or just one of the “call when no one else has drivers” agencies they use.
I would be careful about jumping from a job in Jan to March as agencies tend to be slower around now, so make sure you can afford to run on 3 days a week or less just incase. As mentioned above, agency promises are worthless partly because the client (DHL) can drop you like a stone depending on work required. Getting your foot in the door is often the harder part in this job and a painful part although its worth it longer term as long as you enjoy the job.
COOP sometimes take on new passes and I wouldn’t be surprised if NHS Supply Chain would too. UPS might be worth contacting as they have a depot in that area just down from NHS. COOP run on a lot of agencies, but they use ADR agency as the main one - admittedly they might only take you on LTD instead of PAYE and it also helps if you understand “Polskie Sklep” but I’ve gotten in there with other better agencies too.
Out of interest, which agency are you with?
Go see what they say but just take anything the agency say with a pinch of salt. Plus remember, if they are sending you for assessments then you’re likely losing money on your current job and it’s all for their benefit. A decent agency should be able to find you something especially with a pallet company for example. Actually they might send you to Taylors - that’s an eye opener especially as at least one agency wagon is manual gearbox.