200 Westminster Bridge Road
2. Re: Heathrow to Park Plaza Westminster Bridge Rd
Assuming that there are no engineering works that weekend, you can get from Heathrow to the Park Plaza and back on the tube quite easily. Take the Piccadilly line from Heathrow to Hammersmith. Walk across to the other side of the island platform and take a District line train to Westminster. There are lifts and escalators to street level. Leave the station by the (main) Westminster Bridge Road exit - you'll see Big Ben right in front of you - turn left and walk across Westminster Bridge and past County Hall to the hotel.
On Sundays, the first District line train westbound leaves Westminster at 7.10am, with another 10 minutes later. Make the change to the Piccadilly line at Hammersmith and you'll arrive at Heathrow T5 about an hour later at 8.11am (T123 a few minutes earlier), in time to check in for your flight.
The scheduled tube engineering works are available at the link below, or interactively on the TFL website. A quick browse through the next 2 months shows that the only weekends that your journey would be affected are May 29th (Spring Holiday), June 12th. Even then there's an alternative route from Waterloo with a step free change.