Hows things fella, all ok? 
Edit : Just re-read your post bob, you say 'all' payments...
I'm not sure if you can offset 100% if i'm honest, think its a certain %
Speak to your accountant they'll give you a more exact answer, but you'd be better off buying a van on pay monthly than leasing imo.... Its around the same a month, but if you've brought it you can do what you want with it (i.e bolt tank to floor etc...) without worrying if it voids your terms of lease, and at the end of the payments you've got an asset to sell if you wanted to....
I doubt you will get away with claiming the total repayment against tax with the Receiver.
In the case of a business where the van is a pool vehicle (Post office is one example) where it is only used for business purposes with different drivers, their accountant will probably have applied and received a directive from the tax office that they can do this.
As a sole trader you will have a hard time trying to convience the tax office that the van will only be used for business purposes 100% of the time in the period you are renting/leasing it. You will have to convience them that the van will never be used to go to the chippy at lunch time etc ever. This is the reason why my van is 90% business and 10% private use even although I own a private car.
So in this context, not all of the rental is tax deductable.
Bolting a tank in the back:
A finance lease doesn't carry the same strict return to Lessor minus fair wear and tear as Contract Hire does. A finance lease is typically renting a depreciating asset that will have a value at the end of the lease. That value is referred to as a final balloon payment.
At the end of the lease you can trade the van in for a new one and the trade-in should cover the balloon payment. If it doesn't then you are responsible for the shortfall. You could also take out a loan or other finance and continue to 'own' the vehicle. You could at the outset choose to increase your monthly payments to reduce the size of the balloon at the end of the rental period. If the van was being leased to a building firm, then the finance house agreeing the lease would more than likely refuse a final balloon payment - the final payment would complete the agreement apart from a 'paperwork' charge that might be lieved. So their monthly lease payment would be more than a florist for example for the same van.
In reality, it makes no difference whether you own a vehicle by financing it on HP or lease. At the beginning you start with a new van that costs "X" and after 3 years with "Y" mileage your van is worth "Z". The difference between "X" and "Z" is depreciation which will be the same no matter how the van is financed.
Vehicle depreciation isn't linked to the way its financed.
Leasing is a way of getting the user into a vehicle replacement cycle. Once your 3 or 4 years contract is complete you start with a new van. It is a way of keeping business vehicle fleets updated/fresh and applies to the sole trader as well.
If you take a van on HP, you will usually pay off your HP agreement after a maximum of 5 years. You then keep the van another few years only to find that in the end you will most likely have an unreliable old banger with very little commercial value.
During the length of the HP agreement, the van is registered in your name, but you don't own it. The finance house does and only finishes 'having an interest in it' when the finance is complete. So what's the difference between leasing, contract hire and HP. Nothing. It's how you look at it.