Here’s one for you boys, I currently work in despatch, 38hrs over 4 days mon thru thursday, this gives me the option of working fridays and sometimes saturdays trunking for a blue chip company, as long as I get my weekly rest and keep to an ave48hr working week is there anything my current employer can do to stop me doing this 
Nothing legally they can do - you can work for whoever you want.
You may have a clause in your employment contract that forbids additional jobs but this is mainly limited to not working for businesses that your employer competes against.
Legally they could stop you from taking another job if your contract of employment says you can’t or the second job will in some way interfere with your full time job, apart from that you can do what you like.
Remember ,your boss hires you,he doesnt own you.
quork:
is there anything my current employer can do to stop me doing this
Depends entirely what is written in your contract of employment. If there’s nothing about doing other work etc then you’re good to go.
Does the 48 WTD average apply to both jobs added together in this case?
None of the above! 
you’re an EMPLOYEE of your bosses company, but you can be self employed too! 
as they’ll be paying your stamp/ tax and Ni, but you too can have an accountant to do YOUR BOOKS FOR HMRC PURPOSES! 