I own a domain which is parked with Heart Internet.
They do email forwarding. I forward my name@toys19domain.com to my hotmail address.
I keep getting email spam from the same IP as toys19domain.com.

Bare in mind that unless you have a hosted server your website will be sitting on a shared server with other hosts so it could be one of those that is spamming.

Probably because the hosting company have a hand in it somewhere along the line too.

Its a good thought, have you also considered that they may be spoofing you?
They can put your IP address in the header, i.e. forge it.

right, sussed it. It is a forged header.But the header also shows where it really comes from, so at least it shows my domain is not hacked/compromised.