It's not clear why you would use a journal for this transaction,as it seems to be a credit purchase.
In that situation, use the accounts payable module to enter the invoice against the supplier. If your client is VAT registered, enter the VAT separately. The entry posts as
Dr Computers £1,500 Dr VAT £300 Cr Payables £1,800
If not VAT registered, then post the whole value to the asset account as £1,800
A journal should really only be used if the original entry was miscoded.