For some purposes, in particular to get from T5 to the Compass Center for the Hyatt Place (former Arora) and the Sipson Way/Sipson Road junction for the Leonardo (former Holiday Inn) you can also use this number 7 service: https://bustimes.org/services/7-slough-heathrow which is also included in the Heathrow free travel zone. Bearing that in mind, direct bus is even more likely to be better than trying a train/bus combination.

But it does take a slightly different route to the 423 and at the Sipson Way end it uses a different stop so it's not a direct replacement going the other way. I have found it works quite well.