My suggestion would be to curtail the evening M1 runs at Washington Galleries, and interwork with a short service 8 between The Galleries and Chester Le Street, but serving Fallowfield Way and Fatfield Riverside rather than straight down Biddick Lane. The service would accept through-fares to Fallowfield Way, so that those currently served don't lose out.
Below are the current departures/arrivals/running times for each.
The Galleries to Heworth (M1)
D: 2038* 2117 2147 2217 2247 2317
A: 2104 2134 2204 2234 2304 2334
26 17 17 17 17 17
Heworth to The Galleries (M1)
D: 2008 2038 2108 2138 2208 2238 2308 2338
A: 2029 2059 2129 2159 2229 2259 2329 2359
22 22 22 22 22 22 22 22
The Galleries to Chester Le Street (8)
D: 1859 1918
A: 1918 1937
19 19
Chester Le Street to The Galleries (8)
D: 1822 1852
A: 1842 1912
20 20
As you can see it would be quite comfortable to run such an interworking service by increasing the PVR to 2. The outward M1 trips from Heworth to The Galleries at that time are still quite heavily used, although the opposite way not so much so. Hence the difference in running time. I do believe there's enough people use the Heworth to The Galleries section to more than cover the cost of running at a PVR of 2.
Any thoughts?