This is what I came up with:
LowerBound, UpperBound, count(1) AS CustomersInBound
cast(sum(subtotal_invoiced)/20 as unsigned) * 20 as LowerBound,
(cast(sum(subtotal_invoiced)/20 as unsigned) + 1) * 20 as UpperBound
sales_flat_order AS sfo
sfo.status = 'complete'
AND created_at >= DATE_SUB(date('2016-12-05'),INTERVAL 1 YEAR)
AND created_at <= date('2016-12-05')
GROUP BY sfo.customer_id
) as T
Group by LowerBound, UpperBound
The limitation is, if someone hasn't purchased between a boundary, this boundary will be omitted from the query output.
E.g. If no one's total value of purchases lies between £180 and £200, the Boundary £180-£200 will not be output with zero customers.